While the reference point ( datum )for machining seems plausible, I don't think that is what it is for. A datum would be a point ( Vertex ) not a line. the inside corner at the pistol grip attachment point would be a great datum point. The notion that it may be an index point or tab for a machining fixture doesn't seem right either, the mark is too small to be precisely placed into a notch and not close enough to an edge to be accurately lined up with a corresponding mark on a fixture. And as for a casting mark, why would that particular casting mark be left on the piece when great care is taken to remove all of the others? And why would that mark be left on ALL of the uppers that are made no matter who they are made by? :confused:
Vince

