well 74 had the same controls ( inside with no MAX A/C setting ) and a different system . the 74 has the VIR or valve in receiver system which eliminated the poa valve and replaced it with the vir, which may have relocated the vacuum accumulator again . if you notice in the 74 the poa valve is gone . 73 is weird as its A/C system has a lot of 1 year only parts . so anyway, back the OPs qustion, the dimples are for the vacuum accumulator for 70 - 72 ???