most likely r134-a if it has screw on connections it would be r-12
Freon is R12. Your 1997 Chevy uses R134 Refrigerant.
R134a
It takes r134a refrigerant.
R-134 should be the refrigerant as R12 was banned in 1996.
Chevy switched from R12 to R134a for the model year 1994. Your truck should be R134a.
According to my repair manual the refrigerant is HFC134a.
R134a
They were still using R-12 at that time. Check with your local a/c technician to see if it's still available or if you need to update to a newer refrigerant.
R-134A
r134
134A
R134a