deskthority

It all looks right to be this switch except that the top has the ring and it has a full cruciform mount instead of just the three tabs. I believe it came from the keyboard on an Apple III. It also has gray tabs on the side and a keycap from my SKCC Greens fit (although they are very snug). Are these known to have come like this or is this a special design (e.g. Apple III keyboard)?

The top bit should be a keycap adapter. I've seen it on another Apple keyboard (but black):

http://deskthority.net/keyboards-f2/alp ... ut#p209494

Ah okay. Seems odd to me because I couldn't remove it (not easily at least).

How large are the corresponding keycaps?

I think the keycaps were originally designed for SMK or some similar switches, http://deskthority.net/wiki/Category:SM ... erivatives

The keycaps from my vintage SMK don't fit. The cruciform is too small.

I have a keyboard from an Apple IIe with the same caps and adapters. Caps from other Apple IIes (SMKS) fit perfectly.