From my phone calls to them, Cherry seem to call the nice coloured part of their switches a "plunger" (maybe that's not the right word there, so if that totally doesn't make sense, please inform me. I'm calling them in German, so I'm not entirely sure how to translate it. Plunger seems to be reasonable, though). Interesting enough, they can't do much with the colours, they're much more accustomed to actuation characteristics or model numbers; so, for example, they'll be much happier when you talk about "click actuation characteristic switches" than just about "blue switches." They'll be really happy when you start talking about the nice MX1A-E1JW switches, though. Talking about switches, don't. They're switch modules.
Now getting slightly more on-topic: Naming parts of a Model M is hard. The problem here is the stem basically is an important part of the switch; the switch won't work without the stem, so the real keycap is the removable part on old Model Ms. On new ones, keycaps don't really exist, as you're typing on the actual switch itself.
When I talk about the Model M's receptable, I usually use "pot" in German. "Well" might be a better alternative in English, as is done in the field of quantum physics, for example ("potential well" is "potential pot" in German). If you want to be more precise and technical, I'd suggest guide, key guide or cylindrical key stem receptable.
-huha
