I'm genuinely curious, what sorts of things were these boards typically used for? Industrial automation and control for things that needed more horsepower than a simple PLC, or what? It's very cool to see these weird, obscure things getting emulated!
Tons of things used them, instruments, lab experiments, radio telescopes, :-)
The main source for information on these boards nowadays are laboratories that have stuff running (still) that they're trying to support.
There are bunches of boards on eBay, mostly with insane prices because of the market for spares to keep this equipment alive.