The "mark" variations are to do with the way that the pins through the cation are secured. On an original GP they are held by retaining screws that have to be undone.
In a "Mark II" by circlips.
There are also Webley assembled GPs after they took over Greener. Some will have used Greener barrels with a "knox form" others a standard shot gun barrel with no "knox form".
These can be found with and without the circlips.