Thank you craigslist.
$4 bought a clean, working '96 Lexmark Model M just for bolt mod practice. Ironically it's only missing one rivet. Too bad, I'll have to do a lot of chiseling.
This unit is in great condition. It's also a great guinea pig ... because it's rubber dome.
The rubber dome M is constructed very similar to the BS version. It has the same curved metal backplate, held by the same plastic rivets to the same hunk of black plastic that forms the matrix of key wells. It appears that the differences are limited to the innards of the rivet sandwich, and the key caps.