If there is a smoking gun to be had, here, I think it’s to be found in the shell shapes/dimensions. Only so much can be explained away by the trimming of the shells. The key areas for variation would be the width of the shells when put together, the shape/height of the trigger box after the twist-socket was installed, and the holes/positions for the drop-in parts like the rear fins and side knob. The hole for the latter, I presume, was already marked on the master shell pattern, and the former would surely include the B&W/midgrade-style ribbing before being cut out and replaced with the metal fins. Everything else—the the shape of the P1 cradle, the forehead tower slope (or lack thereof), the side ribs, etc. SHOULD be identical, right?
It can be assumed that one set of P1/P2 shell masters was used for the initial batch of hero phasers. It can also be assumed that the same master was modified and recast to create the midgrade mold, with its attached, simplified handle. The surviving midgrades still retain evidence of the original, black-and-white hero master pattern (such as the original ribbing on the rear of the P2 and the scribed guidelines for the two small half-round power meter buttons on the B&W P1. So, it’s no stretch to say that all the hero and midgrade phasers we know to be authentic and original came from the same molds (or a modification of those molds).
If the auction piece’s shells vary significantly from the known details of both the heroes and the midgrades, then the only logical result is that, if suddenly requiring new props beyond the four heroes and the midgrades, they either reused the existing molds, or needed to create (or recast from the existing props) a NEW master for casting shells (resulting in some dimensional variations), which seems…unlikely.