Hi Cadfish, here are the pictures.
I think this thing is patented and only one company makes them. Not only they're expensive, but the service life is limited to something like 4000hrs.
Very simple device, works on a principle of controlled deflection.
1. input bearing called wave generator (pictured left)- oval in shape, that's where the motor goes
2. Output side aka flexible spline (pictured right)- this part turns inside the housing
3. Housing with teeth on the inside aka rigid circular spline (not shown). Housing can be filled with transmission fluid, bearing required on the output side.
The trick is to buy complete gearhead with sealed housing and output bearing- the most expensive state. Medium size 4-5" gearheads go for $1000 or more (company direct). Ebay has them too, unit without the housing goes for $50-100. Complete gearheads attract bidding wars and go for hundreds.
Here's the big one, 7" dia 1:160 ratio. It's got massive crossed roller bearing on the output side and can easily cost over $2k.
More info here
http://www.waltmusser.org/HD.htm