The Wellworthy piston is an unfinished forging for a late Merlin piston. As an aside, pistons were good for scrap-men, they are quality alloy and, if just boxed, they have no ferrous to remove-just toss ’em in the smelter. Partly for this reason, WW2 German pistons are rare, apart from those from crash engines, they were all smelted.