If you look at the British Proof house charts which gave gauge dias from 1-50 inclusive & then from .450" down to .300" in .010" increments, a .410" dia is a 67.49 gauge. This size can also be found marked on pre 1912 German guns (Smooth or Rifled) which would take a .410" plug gage but not a .420" one.
The 36 gauge is normally listed "Rounded Up" to .506".
12mm is .472" which would fall around mid point of the chamber of a 2½" .410, it's certainly not the bore. 36 gauge is just a little under rim dia. Certainly not much rhyme or reason for such terminology, but both terms have certainly been used.

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