Re: Not sure what they used...
"I don't know that any off the shelf commercial CPUs of the time would be sufficiently hardened against radiation damage."
Unlikely. However, there were 4-bit 'ALU' devices (which could be daisy chained to form 16 or maybe 18 bit words, as needed) available in TTL at the time. DEC used them in some of their computers, actually. They weren't really fast but a 250khz clock would run them just fine, most likely. A little bit of microcode and they'd form a CPU on a single circuit card. Using flat packs, maybe even CPU plus RAM.
but yeah they'd have to ruggedize it for space.