I did read, I dont know whether it was advertising bumf or not, that they had to modify the production of the later Mark 1s due to them becoming very close if not exceeeding the U.K. m.e. levels.
I do remember on the rare occasions I put mine (Mk 1 .177) over a chrono (which was at M/c Air Guns - not even all gun shops had a chrono in those days) it was very close to the legal limit.
Great rifles - pity about the trigger and barrel wobble.

ATB
Ian