AJ
Coulda been any of the POs that changed it. Whoever did didn’t put a sticker on it and of course the other(s) didn’t or even cross through with a marker. Not unusual
But the fittings are definitely R134.
N2 is nitrogen. It’s used instead of air when pressure testing. Air can react with old Freon and form acids. You will have to evacuate the system afterwards anyway but N2 is what is normally used
The 134 swap kits supplied back in the 90s never included the TXV, so it’s going to be for R12 unless professionally done and I think the sticker, or lack thereof noting the change says something there.
R134 in an R12 system was never really great. Between the TXV/oriface, evaporator and condenser mismatches, you get something that works but never as good as one designed for the correct refrigerant. I did the swap in my ‘91 with a product called Freez12. It was a mixture of 134a and at least one other component that was designed to come close to the adiabatic curves of R 12. It was ok,not great, and when you use a mixture generally you get changes in the mix when leaks occur. You have to evacuate it all and replace with new. You can’t top it off