[kens]

An APL keyboard typically wasn't exactly a custom keyboard, but a standard keyboard with APL characters accessible through modifier keys. Often there were adhesive labels on the front of each key showing the special characters or you could get custom keycaps that had the special characters printed on the front. And of course many of the special characters were created through overstrike rather than separate keys. Way back, I programmed in APL on a DECWriter that had support for APL characters.

[cmrdporcupine]

I have a keycaps set imitating the classic Space Cadet keyboard (from Symbolics Lisp workstations). The Space Cadet had APL tops and fronts [1], but these caps only have the tops presumably because the cost to get keycaps printed on the fronts would be way too high. Frustrating though. I've thought about getting the rest printed on clear plastic adhesive.

[1] https://en.wikipedia.org/wiki/Space-cadet_keyboard#/media/Fi...