I inspected a 1916 Knabe grand which the owner wants to give away.
The key button is quite thick and has a flange incorporated into
it just behind the balance rail pin. The back half of the key is
split diagonally with the top half pinned to the flange.  The
bottom half rests on the back rail while the top, hinged section
extends beyond for the stickers to lift.  I've not seen this before.

The player action has been long gone and the two sections glued
together.  The piano is in marginal condition but the key set was
interesting.  The owner needs it out before the end of March, so
act fast...

Regards,
Jon Page
Cape Cod, Massachusetts