Aside from what's engraved on the reciever, Look at the arse end of the forend.
The Mk1 will have a rivited bracket.
The No4 Mk1/2, 1/3, and Mk2 will have a screw and nut in the same place.
Since the later version has the trigger hung on the reciever, it was necessary to have the forend open at that point. The screw and nut must be removed before removing the forend or it will foul on the reciever mounted trigger assembly.