There has been a few attemps by 3 Piano makers worldwide in the recent history but none of them are copies. The number of them is extremely few. From a long time it was wrongly said that this piano must have been altered ''completely'' by Anton Walter himself and then partly by Mr. Rück in 1930s.
It is simply not true. It took me 3 years to understand this and making the first copies.
It was possible through studying all early Walter pianos in detail by visiting the two most important ones namely : Mozart's Walter and the Eisenstadt Walter (in Haydnhaus Museum) which did probably not belong to Haydn though.
With the help of Christopher Clarke and Albrecht Czernin more detailed photos of some other early Walters that had never been studied made it possible to do many comparisons of his techniques and stratedgies for his Phase I and II periods. These were the most interesting pianos he made to my opinion but apart from personal opinion and understanding of sound and resonance as a musician the later Walter pianos that are being copied here and there are not by any means representative pianos of Mozart's time due to the extremely fast shifts of concept of the sound developed by Walter. He was doubtless the leading piano maker in Vienna and soon in the german speaking areas but also Italy. His pianos were more expensive and his little developments of the action upon the already perfect Stein action made him proudly the father and inventer of the so called Viennese action.
Before the Viennese action (Prellmechanik with escapement and back-check with brass kapsels) he did continie to make small changes in the shape of the wooden kapsels every time and to my opinion logically Mozart must have bought the Instrument from Walter very early as it was made with such wooden kapsels and I am 99% certain that Mozart must have naturally become interested in Walter's modifictions so he must have given his piano to Walter to rebuild the action (around 1785 as Walters pianos already were being ''mostly'' made with brass kapsels and back-check) and politely asking Walter (who was a bit rude to Beethoven I believe!) to even make the piano's dampers work with knee-levers like that of Stein known very well to Walter and Mozart since they were both kids!
So Walter not only renovated the action but also respected Mozat's wish to only drill 2 holes so that through a simple mechanism dampers raise when he raised his knee.
Walter made both Mozart's piano (not knowing for whom he makes) and the Eisenstadt Piano as ''prototypes'' from one walnut tree! it is clear because the lids have exactly the same shape and it can never happen in nature otherwise to cut 2 different trees and their figures look the same!

Walter made both pianos also nearly the same with extremely little difference in dimensions and string lengths. Few other things as well like the bentsie is 1mm thicker on Mozart's piano.
Mozart's piano has no enameled plate beautifully framed in a brass bezel bearing Walter's name but Eisenstadt Walter has this which means it must have either stayed with him for several years as a playable example? until he sold it in a time as he was allowed to sell and put his name on it proudly like every other of 9 other piano makers in Vienna. Or he had sold it to another composer or musician who also asked him to renovate the action a few years later (same story with Mozart's piano) because both pianos have exactly the same action, same kapsels (with 3mm thick kapsels) and relatively heavy hammer heads. But this one's dampers was not updated by him but by a very strange tasteless person many years later before leaving it hopeless in a barn near Austrian border in Hungary!

In any case the fantasy of our previous experts that Mozart's piano must have been made with Stoßmechanik and renovated by Walter in ca.1810 is not correct because of various reasons. Some of them explained above. The other reasons is because Konstanze Mozart writes in her letter to her son that Mr. Walter came here (not saying the piano was sent to him for full renovation!) and was releathered and maintained.
We all know that it makes sense to ask the piano restorer to maintain the instrument for a few hours and if not releathered in the recent 15 years better to use fresh dry leather.
Nobody restored the piano until Mr. Ruck repaired some cracks on the soundboard, luckily never took the soundboard out because there was no need for two reasons: the way it was made did not make it necessary to do such big repairs and besides it was like a holy relic in the family, at the end was too out of fashion that could be kept really only as a holy replic until it was handed over to Mozarteum in the middle of the 19th century and they also stored it in their show-room and didn't do anything until 1930s. Mr. Rück unfortunately threw out the knob (of cow's horn) for the moderator handstop and used a new one, then removed the 3mm nut and put a more steady 5mm high nut which is a pity but not a terrible issue.
He did not plane the blocks under the underboard (bottom rims) with an angle! on Eisenstadt piano which is les modified that Mozart's piano (with the original knob, nut and untouched bottom rims) they are square and simply. On Mozart's piano the bottom rim along the spine looks like being removed. We can be sure all these were done once Mozart asked him to do the few things explained above but it seems Walter who understood that that piano is such an important representative of his work around 1785 he found it ''better looking'' to spend around 2 hours to plane those rims because the bottom of the piano is not nice to look at in concerts. We can very well imagine those 18th century people who were around 10% shorter than us and had a much lower chairs than 10% compared to our chairs could easily see the rims.
So the whole package of these changes must have cost Mozart something (he was rich unlike what unreal stories like that of Amadus make us think!) so he must have paid or maybe Walter didn't ask him as a kind gesture so that Mozart speaks highly from him what we call ''Mund-propaganda'' in German.
This scenario is the only scenario that makes sense and is based on both common sense and what has happened to the piano considering all the history of the piano unlike we want to ''fantasy'' that all this must have happened by a ''Person'' in Milan where Mozart's son kept the piano and played on it during those years he worked as an ordinary employee of the state (Beamte).

We should anyways remember that neither Konstanze was rich nor her son and apart from all this: the piano was with its 5 octaves and old-fashioned sound simply out of date in the romantic period for the majority of musicians, even Mozart's other son who was a professional musician and composer didn't take this piano! in the 1790s after Mozart's death appearantly Konstanze Mozart thought of selling it but surely didn't do it and prefered to sell the manuscripts.
She also writes in that letter to her son that this piano whose maker is Walter and sounds better than that time (on this subject there is more to be discussed) had buyers but I said no to all and kept it.
In Viennea there were some attemps by probably frauds who tried to sell their piano and clavichord as Mozart's instruments! it was easier back then to do deceive people. It has always been the case for violins, even today some of the cheapest violins bear the label of Stradivarius!

Alfons Huber guessed that Mozart's piano must have had Stoßmechanik due to some carvings in the body of the keyboard frame which I have to reject.
He also shows pianos made by 2 other masters in Vienna (Kober and Christoph the latter taught Walter before he became so famous, no one took him serious back then) those examples show that before Walter the action of few pianos made in Vienna 1770s was Stossmechanik which looks like the English grand piano action more or less, also to Silberman or Christofori. That is true but those signs in Walter's action (shown here) show very simple things and close the door for any kind of detective fantasies. Those are only for making more space for the hammer head and for the damper stool.
It is rather strange and funny to see that the lowest key (FF) was made a bit too narrow by mistake and therefore he had to make room for the lowest hammer and damper-stool. This is another sign of this piano together with its twin for being called ''prototype''.
Another reason to be prototype is this: both pianos were made more or less from the same drawing (though there must have been no drawing at all!) following the same ''thing'' on his head because both show he thought of making these pianos bichord (with 2 strings) but then he changed his idea and added the third strings at the treble. His much older colleague in his fatherland Johann Andreas Stein did vice verse, he made first bichord, then thought of making triple chord at the treble and then came back to the bichord towards the end of his life.
Here Walter short after making the piano added the third strings in a funny way what in German we call ''Basteln''!
He had to add the third row in a miserable way and put some helping pins to give the third strings a bit of angle so that they can sound. All other later instruments have proper looking tuning pin holes following a logical shape that makes sense.

With great interest I stand for these two early Phase I Walter Prototypes despite of few updates and mistakes he made. These sound best for the music of 1770s and 80s but even 90s. The piano sounds without too much power soft yet with great resonance and projection, stands perfectly with an orchestra espite its thinner strings and softer construction compare to always heavier and heavier Walters. Beethoven's music and technique requires them, again we need more the common sense to come to these conclusions than our favoritism and preference.

This Phase I Walter has the benefit that the soundboard grains run diagonally and rest on a so called ''Full-height frame brace'' which makes any repair later much easier for the restorer. Diagonal running soundboards have their own characteristics in sound and crack more often than common ones but again: the repair is very easy and at hand without removing the whole soundboard. The so called dead-soundboard is there rather as decoration.
The weight of the piano is less than 95Kg which makes it a much easier fortepiano for gigs! comparing to later instruments that is definitly a bit too heavy even for 2 people.
Due to only 700kg string tension the piano has less stress and more resonance, less trouble for the right cheek to raise in the air.

One of the main reasons we can be sure about Mozart's piano having had wooden kapsels and no back check is because there are several examples (not recorded in any book or studies yet except for this article and a condition report of a Walter in an auction in 2021 by Clarke) these examples were all made within a few years after the Phase I and are in original condtion except for the Walter in Technisches Museum which was probably altered by again Walter himself. They all have wooden kapsels except the Poznan Walter being strangely dated 1789 but still having wooden kapsels while the approximately 1785 Walter in GNM probably the earliest Walter known having the so called Viennese action (brass kapsels and back-check).
According to Christopher Clarke back-check only works well if the kapsels are from brass rather than wood because the bever-felt bushings (no matter how durable they are till today) could have gone out of order much earlier if the hammers were stopped by the checks all the time.
So this must be very correct because Walter, Stein and most others understood it and did the same except for very few examples (2 examples in Radbon Fortepiano Collection and a Karl Benedinkt ca.1785 Fortepiano made in Graz).
So because there is no evidence, any signs of having a frame that holds the hammers in the Stoßmechanik style on Mozart's keyboard frame, we can only think of Mozart's piano being made with Stein's action seen (as explained and illustrated above) on multiple Phase II examples.
Phases can be defined by comparing the bridges, shape of the piano, material of the bentside and lid, action, inner struction and more factors.