Diver and Chief
I don't know if this would be of any help. Years ago I played Myst and I remember in the building with the all the books, fireplace and map there was a book that had the diagram of a piano board.
Facing the 3 shelf bookcase look on the middle shelf on the left side. There is a big blue book. Look through the blue book and towards back there is a picture of a key board with numbers.
I don't know if this is the keyboard you are talking about, but it is a good diagram.