No conversion needed as you don't need to do anything to actually FIT a Mk1 gas cylinder to a Mk3 gun. They are both the threaded sort. Indeed, the first Mk3 guns were fitted with Mk1 gas cylinders (but not for long I hasten to add!). This is where I was confused.
The Mk2 guns had the press-fit gas cylinder and ain't not interchangeable with the Mk1 (or 3) guns