Most 94s and 95s, except ZR-1s, were part of GM's "early rollout" of OBD-II diagnostics.
The ECMs in those car have full OBD-I systems along with "partial OBD-II capibillity." These systems' OBD-I works like other 81-93 systems. The partial-OBD-II was there only for the program GM was running at the time which collected data about the field performance of their OBD-II system. Any OBD-II codes set would not turn on the SES light but would store a code. Anytime one of these cars came into a dealer, the dealer read the OBD-II side of the system and, if codes were set, sent a report to GM. At the time, the only equipment which supported this partial OBD-II capability was the dealer scan tester, the Vetronix TECH 1A fitted with an OBD-II auxiliary software cartridge and a 12-to-16 pin adapter.
As for OBD-II hardware, all 94-95s had the 16-pin DLC and had some of the sensors, actuators and wiring harness which would be come standard in 1996.
The pin 4-12 connection is for CCM diagnostics and the H48 code, which you saw on the IP display, is a history code. 48 sets whenever the ECM detects engine reference pulses but no signal from the MAF. As long as the SES is not on and you don't see "C48", then it
s a fault that may have been a one-time deal or is an intermittant that's happened in the past, but is not occuring now. I'd clear codes and road test. If you don't see the check engine light, again, I'd not worry. If the code sets again, then I'd troubleshoot according to the DTC48 information in the Factory Service Manual.
You can clear codes on a 94-95 by either 1) disconnecting power to the ECM for at least 30 seconds, 2) disconnecting the battery for 30 sec., 3) using the clear codes feature of a scan tester or 4) using the CCM diagnostics (pin 4 to 12) code clearing instructions listed in Section 8D of the FSM.