"I think the only rad-hard CPU around at the time was the RCA 1802:"
In this time frame it was this, the 74181 ALU (acting as a 4 bit slice of a larger processor, either in groups as a parallel processor or time shared as a 4 bit parallel, n segment serial processor), with real hard core types staying with individual TTL gate packages. CMOS equivalents were also an option, and the wider supply range was attractive. In fact much of the 1802 design choices were due to the limits of the CMOS process used to make it. The pay off being the ability to run down to DC and still retain state, not something a lot of modern mainstream processors can do.
Just a completely different world from today.