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On the transparant version there are two horizontal lines slightly further apart (38.5 mm), as indicators of the 6x4.5 cm format. That these lines are a bit further apart will not mean anything, just the way they're made. ;-)
The opening is the same size as in the black mask.
The outer dimensions are 58.65 mm x 58.5 mm, with bevelled corners (bevel length 3 mm) and two 7 mm long and 3 to 4 mm deep recesses on each side to accomodate the focussing screen catches.