It's a unit number button for an RL01 or RL02 disk
drive.
The different numbered buttons have different kinds of
"fingers" on them that are detected by the drive so it will
know if it's drive 0, 1, 2, or 3.
I actually could use a "3" button if someone has one. I have
two RL02s and an RL01 on my system. I plan to add a
second RL01 so I can copy images directly from one RL01
to another, but I need a "3" button.
If you know someone who has a "3" and doesn't want to part with it, I might
be able to duplicate it. (White is an easy color :-).)
Now, if I could just get the colors right on these dang switch handles...
Vince