Proof charts which show the smaller gauges show a 36ga as having a diameter of .506" & a 67.49ga as .410".
I think it probable that true 36ga guns were not in common production at least & someone decided to flatter the little pipsqueak a bit by just assigning it aa nice sounding gauge size a bit smaller than a 32ga & 36 seemed the next logial step.
It did need all the help it cold get, can you imagine going to the gun shop & saying I want a box of 67 point 49 gauge shells.
A pity they didn't actually just build the true 36ga instead, would have been a better platform for a half ounce of shot, not to mention trying to stuff the little thing with 3/4oz. Of course 32 gauge is the logical smallest desirable size for a shotgun other than a snake load for a pistol.
