73 vs 80' OEM ac differences ???

[brimac50]

I know the earlier trucks used the heavy A6 compressors,  ..... and the later the R4 model.  My questions are ... did the A6 trucks use a TXV, instead of a fixed orifice tube? Was the compressor controlled by a cycling  pressure switch, like the R4's?  I'm kicking around the idea of installing an A6 on my 80', and need to know how GM designed the "earlier" systems.  Appreciate the help. Thanks.

[Lt.Del]

My 79 has the long compressor style.  Has a pressure switch?  yes, two green wires attached to the switch, switch screwed into the dryer.

Thermal expansion valve versus the orifice, I am not sure, never had my 79 apart, but, I don't beleive there is a expansion valve on it.  
I know my 91 sub has both, valve on rear a/c, orifice in line below the dryer.  

[VileZambonie]

The early years used a TXV - in 1980 the CCOT system with the A6 will work just fine.