The early 1990s is when the change from R12 to R134a took place as it was mandated to be done by 1995, so it could be either one.  Two ways to tell which refrigerant you have:

1. There should be a tag on the unit indicating the type.
2. R12 service valves (Schrader valves) are threaded to accept the gauge set.  R134a service valves are "quick connect".

Enclosed is a picture of the two styles.

The service line (vertical tube attached to the compressor) appears to have a threaded dust cap on it, suggesting that it's an R12 unit.  Not unheard of, but 23-years is a long time for a refrigeration unit to be still fit-for-duty.  Might be time to replace rather than repair.  The Waeco CU55 and VD07 should be very close in size and capability to what you currently have.  But measure things at least two times to make sure.....

For what it's worth. I put a brand new Alder Barbour unit in 3 years ago and it has Schrader valves on it. I would go by what the tag or manual says and not by the valve type.