12mm =.472+", 78.9mm = 3.1", bore is marked @ 10.5mm= .413".
Looks as if they used the old 12mm designation for a .410 with 3" chambers.
12mm is a size that falls within the chamber dimensions, not the bore size as shotguns are normally labeled by. 36 gauge was also sometimes used but this is also incorrect. A true 36 gauge is .506" in diameter while a .410 bore is a 67.49 gauge.

Miller/TN
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