Essentially a PM5
The PM5 + PM9 are essentially the same ECU. I think one may have been for the 4 speed and one the 5 speed, but I'm not sure. (blundar)
The PM9 that I posted was from a 4 speed HF model Civic. You may be correct in your thoughts. (Warrick)
The PM5+PM9 used two distinct designs (kinda like the PG7) - the 88-89 Use an 83C154 while the 90-91 use a 66201. The 90-91 processor quadrant looks a lot like an OBD1 ECU.