OK, here is what I know about these ECUs.
The PT6-A51 I have here has a 66911 MCU and uses an external 256k PROM.
I got an ECU out of a 1994 Prelude SI with a 1.6 DOHC and it has a PK3-A00
and it also has the OKI 66911 MCU and external 256Kbit PROM, plus has the 74HC373
-- sloopy - 16 May 2005