My best guess is that the initial hobs and mold cups were machined to 1-5/8" diameter (1.625") with plans to machine the excess flashing down to a 1-1/2" (1.5") final product diameter (some older TRKs are actually this size). And at some point, less material -- only the minimum amount necessary -- was removed during the machining/grinding phase, resulting in a somewhat 'standardized" 39mm (1.53543") final diameter.
Just a guess, though.
@gmunny @PAZ @David Spragg ?
But they aren't the same size -- US silver dollars are 1.5" diameter (38.1mm).
