This is what I can work out, but I'm not certain:
Mk1 shorter stroke, no anti bear trap, unshrouded barrel, no chequering, blocky looking stock
Mk2: shorter stroke, anti bear trap, chequering, still fairly blocky stock
Mk3: longer stroke, anti bear trap, shrouded barrel, fish scale chequering, more curvy stock
Then the HW77 got the curvy stock treatment as well...bringing some kind of symmetry to the development of these guns - inspired by each other, I guess!
What do you think? Have I got that about right?
Oh, and which ones do you prefer? In the link I posted previously, the shooter thought the Mk3 was more hold-sensitive than the shorter stroke versions.