Of the three markings; .410/12mm/36ga only the .410 is correct. The .410 is named for its bore diameter of .410". 12mm = .472" & 36ga as mentioned = .506". The actual gauge of a .410 bore is 67.49 as shown on the gauge charts which go down to the smaller sizes. Many charts stop with 50gauge @ .473" but on those which list the smaller sizes these are carried out to two decimal places on the gauge numbering & the usual three decimal places for the diameter. These sizes actually drop in .010" increments & give the calculated gauge number. 50 gauge or larger are all listed in whole numbers.
