I stumbled upon something moderately interesting today.
To my understanding, the model 31 is typically subdivided into the '31, '34, and '41 pattern guns.
I was switching a magazine plug from one gun to another today. It's an actual Model 31 plug and it has a paper instruction label wrapped around it. The writing is worn down, but there are clearly two sets of instructions; one for '31 & '41 guns, and the other looks like it says it's for '36 guns.
Has anyone else noticed this or am I blind?