I have one of the Remington semi-hammerless singles in 20ga. With a cutoff barrel at about 26" it's not exactly collector quality but not bad condition.
I can sorta see how they came up with 12mm for the .410, presume this was based on the case size of a 12mm round @ .472" rather than the bore of the shotgun, but never quite figured how they came up with 36 gauge. With a 36 gauge @ .506" bore I suppose it was just figured this sounded better than calling it the 67.49 gauge that it truly is.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra