To elaborate on what Fontyyy said. The PGM started in '87 on the MC18, but the F3 version used the CDI from the RS. '88 MC18 went onto the PGMII. We know the story after that.

As for differences, there are many. As in what they monitor and how they control the ignition timing and RC valve operation.

NF5 PGMII differs from the NH3 PGMII in timing and RC valve timing. There's also wire snips that can be done to alter the timing further. The NH3 PGMII offers one wire snip, the NF5 offers two snips.

I've done a fair bit of bench testing on the PGMIII and it monitors many things such as RPM, gear position, throttle posistion and throttle opening speed. The ignition timing, RC valve control and solenoid control are all controlled by these. Then you "wire splice" it and change it all completely!

For your info the stock PGMIII, spliced PGMIII and NH3 PGMII are very different in ignition timing.