The term 36 bore or gauge is still sometimes used in Europe, particularly in Italy.

It seems to have sprung from a desire to have a “tidy” numerical progression , 20 to 24 to 28 to 32 to “36”, whilst ignoring the scientific basis of number of lead balls of that diameter to the pound represented by the other bore sizes.

Perhaps the gun and cartridge manufacturers felt that buyers would think a 36 bore would have more poke than a 67 bore, and would sell better?