yes, REV A and B are one-time Proms. REV C and D are EE proms.
I remember reading somewhere that a A and B prom can be programmed by zone, so if zone A is programmed (and useless to you) and B C D are blank, you can program what you want on zone B,C,D then somewhere in the radio is a jumper to select zone B, if it is only a single zone radio. interestingly, I've taken apart some of the modules from the 70s and they contain SMT circuits. I thought SMT came out in the mid 80s.
the MX was such advanced technology for its day.
I had an R1801 about 20 years ago and the thing was not built well, it was, in my opinion meant to sit on a bench for its entire life, not be carried around, so much wiggle wobble from inside it.
