deskthority

Were the Selectric Ⅲ's cylindrically-sculpted keycaps mechanically compatible with the earlier Selectric Ⅰ and Selectric Ⅱ's key-stems? Put another way, I'm asking if keycaps from a Selectric Ⅲ can be used on a Beamspring keyboard?

No. I think it was reddit where I ran across the threads that had the info. But beamspring to Selectric III is a no go.

Actually, it's yes and no. The mounting points on the selectric iii caps are off center, but they use the same mount. so you CAN mount them but you can't reasonably use them in any meaningful way.