Yes indeed, Martin is right!
I have just pulled my Mk2 from the rear of my Gun cupboard (it's one I aquired some time ago and I havent looked at it for over a year!
) and lo and behold - it has the same barrel stampings on the top of the barrel as my Mk4 and later Mks have. (never noticed that before) Looks like all the 'parallel' barrels are so stamped then! - I learn something every day!
So it looks as though your Mk2 is a standard one and may well have not been tampered with - one way to check is the muzzle end of the barrel, as the standard Mk2 barrel will have the .22 cal size bore, all later barrels featured the larger size muzzle opening which opens up the end of the barrel a few inches.. (sound moderation/air stripper?)
ASM