Walter, I just came across this thread. Hopefully not too late to be helpful.
I can vouch for the A22/ES-22a/ES-12a/HC-12a refrigerant. I used it (under the last 2 designations) in my '92, and it worked excellently (frigedly
)! I didn't have to drain anything or convert anything. It was alot cheaper than the old R-12 freon, comparable in price to R134a, and able to mix with either. According to one posting on some board I came across, R134a is highly carcinogenic, even through skin contact. I can't confirm that, though.
I ended up using quite a bit of the stuff (And R-12 before I discovered this alternative) since after I replaced one component of my A/C system, another would spring a leak, until I had eventually replaced the compressor, receiver-dryer/accumulator, and the condesor! I found all 3 components used in junkyards. I removed the receiver-dryer myself, but I had a shop do all the installation.