If you are in doubt about whether or not your Air Conditioning System has been changed from R12 Freon to R134a Freon, this guide should help you figure it out.
The Reatta has two fittings used to service the AC system. The "Low Side" fitting is located on the side of the AC accumulator. The other one, the "High Side" fitting, is located on the high pressure line on top of the firewall under the cowl cover.
Both of those R12 fittings from the factory had threaded fittings that can only be used to connect R12 servicing gauges and other R12 equipment. When the system is converted to use R134a Freon a Quick Disconnect style fitting is screwed on over the top of the stock threaded fittings.
You can see one installed in the photo of my accumulator below. The Low Side fitting on the accumulator is the one that is used to fill the system with Freon. The High Side fitting is usually only used when troubleshooting the AC system. Only R134a gauges will connect to the R134a adapters.
Never attempt to fill the system with Freon by attaching a Freon can to the High Side fitting located on the firewall!
It should be noted that there is more to changing a AC system over to R134a than just adding Quick-disconnect fittings to the stock R12 fittings. To do the job properly all R12 Freon and oil in the system must be flushed out before filling with R134a Freon. New compressor oil designed to work with R134a Freon should be added to the system. Also, special o-rings are required at all connections to do the job right.