The rules state that a professional technician changing the original refrigerant to another chemical must change the fittings and label the system. Every different refrigerant has different fittings to be used with it.
MB R12 fittings are similar to US manufacturers except that they use the large Hi side fitting (same as low side) unlike the later US cars which changed the Hi Side so people wouldn't put the wrong gauge on the wrong side.
Most competent A/C shops now have refrigerant identifiers. It might be worth it to get one to try and ID whats left. The main problem is with the oil. If the system has been changed to R-134 the oil used will not be compatable with R-12.
The best thing would be to flush the system, reoil, replace the drier and return to R-12 as the early 124 body does very poorly in R-134; probably do to the undersized compressor that was updated in 1990.
Bosch Master, ASE Master, L1
33 years MB technician