I have one of the oldest living IBM Series M
keyboards with the original IBM APL2 keycaps.
I use APL a lot.
The APL support in /usr/share/X11/xkb/ doesn't
include support for that keyboard.
So I patched /usr/share/X11/xkb/symbols/apl
and /usr/share/X11/xkb/rules/evdev.extras.xml to provide that
support. If anyone is interested, I'll be glad to provide the
patches. (They're against Fedora 33.)