Are the codes on the 1972 cutlass door locks supposed to be the same for the left and right or is the forth digit different to distinguish between left and right?

 

If you are referring to the actual 4 digit key code, they should be the same for both doors... Each 4 digit code is a different six cut sequence for any given key... Sample chart below...

 

Yeah, I was talking about the key codes. The forth digit is different on the locks so I guess the previous owner changed one of the locks at some point and it explains why my key only works on one lock.

 

The RH and LH cylinders are identical. There would be no reason to make the codes different, and as shown above, the four character code specifies one key pattern. A different code specifies a different key pattern (and possibly even a different blank).