I found this at Retro Bristol. This is a plastic cover for the breadbin style C64's that features a few piano keys, and together with the bundled software allows you to "make music" utilising the C64's SID chip
It was initially released in 1982 and the cost at launch in the UK (I think?) was £29.99. There does not seem to be much written online about this peripheral, maybe it didn't sell all that well
Anyway, this one is mine, and here it is
The top and bottom of the box, it's not in the best condition
Front and back
Both the sides
The piano keyboard cover, it does feel very flimsy and that it would break easy. I believe the stickers came with it on a sheet and had to be (badly) applied by the user. The piano keys just press the C64's keys using the top two rows
Some embossed text
This is the software that was in the box, whether it originally came with the keyboard or not, I don't know, there's two cassettes, it says it's compatible with the C128, but the piano cover is not going on a C128 anytime soon
This is the documentation that was in the box
F - Invalid file name, 310:1