After a little more research on the net tonight: it looks like the keyboard in the 4006 was pretty much one of Cherry's standard "B70 ASR" models. The key mechs are known as M7 T-mount, and the caps look like the standard double shot ones supplied by Cherry in their mid to late 70's catalogs.
A lot of the Tek terminals I've seen for sale seem to be missing a few teeth; I guess these caps didn't hold on well enough to weather the decades without a few from most keyboards getting lost along the way. It's probably worth working up some 3D models for these caps. Also, keyboard research turns up some sad, sad, things... Brrrr... :-( --FritzM.