When I was growing up (I am talking maybe 80 years ago) we had a piano (and I had lessons). It was basically a standard issue upright, but with one difference. On one end of the keyboard there was a lever and if you pushed that lever and quarter cylinder descended just adjacent to the top of keyboard. It was maybe an inch in radius and the length of the keyboard and had musical staffs inscribed on it. And above each key the corresponding note was marked. This may have been only for the notes near the middle, because it seems unlikely to be able to do this for all 88 notes–I just don’t remember. This was obviously made for teaching. Does anyone here remember seeing such a device on a piano?
Are you describing a transposing piano?
It definitely was not transposing. In all other respects it was ordinary.
If you google “upright pianos for beginners” you will see a few photos of a separate paper overlay that sits on the top of the backs of the keys and has the name of each note on it. That’s as close as I could find to what you described.