Years ago, GM did not want their compressors cycled off and on for temperature control. Funny though, their dealer installed under dash units, called "CoolPack", did exactly that and AFAIK used the same compressors. They used hot gas bypass until 1962 when they switched to suction throttling valve (basically an EPR-- their terminology, not mine). In 1965 they switched to POA valve which did about the same function. Somewhere around 1974 they went to a VIR (valve in reciever) system. I think they switched all their system to orifice tube and cycling clutch in 1977, though they first came out on 1973 p/u's and Vegas. FWIW, the hot gas bypass line tied ino the outlet of the evap right at the txv bulb, rather than just dumping hot gas back into the compressor. I still have my book from auto a/c class in 1972, though some of it is not in the best of condition. If the OP wants any info, I will be glad to make some copies and mail them. I have no way to scan/Email them tho. Larry