Hi simon
EPA phased out R-12, which I believe took 15 years to that job, unofficial they are available around UK , they are environrant pollutants . The entry of R134A came in around 1992 and thus became controlled . R134A. are known to be used from 1992 onward models as OEM.
Most retro kits did not work as well as OEM systems do now, since 134a requires MUCH less to cool the same amount of air (hence more energy efficient).
The major difference between the two according to my instruction was the size of the tubing in the entire system, r-12 needed larger tubes than 134a, and attempting to use the r-12 "legacy" type evaporator/condenser units in addition to the compressor... depending on how much you love your car, the LS400 1992 model onwards have started to use the R134A as the standard cooling equipment. In short to find whether it is R12 or R134 it is the tubing system which you can easily differenciate.
cheers
mann777