Brian, for comfortable recoil, heavier loads for heavier guns and vice versa.
All Parkers were proofed and intended to be safely used for the maximum service load for that gauge, an example of which would be 10,500 psi for a 12 ga in the 1920's. When SAAMI standards were established for service and proof loads, Parkers met those also.
If a person wanted a light gun he could comfortably carry all day, that's what he told Parker and Parker chose the frame size to match. If he wanted a gun that he could comfortably shoot 100 rounds of trap, Parker selected frame size for that also. Both guns were built to shoot the same 1 1/8 3 dram load. buyers commonly told Parker the desired weight of their gun in addition to stock dimensions.
Yet, all 12ga Parkers could and likely did shoot anything from 1 oz 2 1/2 dram loads to the max at 1 1/4 oz 3 1/2 dram.
May I suggest you obtain and read The Parker Story, which explains all these matters and more far better.
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