The MKI and MKIIs had a shorter stroke length. UK forums would know more about the other differences. They went to a longer stroke for more power with the MK3 if I recall correctly. Alot of shooters in the UK short stroke their Mk3s. Tinbum sells a kit.
When I was looking at them, mk1 didn’t have the notches on the sliding chamber for the bear trap iirc, mk2 has unevenly spaced bear trap notches on chamber and diamond checkering on stock, mk3 has even spaces on bear trap notches and the fish scale style checkering on stock.
The Mk3 has a shrouded barrel and the other internal differences noted above. The Mk1 and Mk two have what looks like a barrel weight on the end of the barrel and the little rectangular piece underneath for the latch to snap into. The Mk1 has no bear trap and the Mk2 does have the bear trap which is the easiest way to differentiate between the two.