Generally the simplest method to differentiate the marks:
Mark 1 - 5 port muzzle brake, name engraved on breech block, no hole in cocking button for power adjuster;
Mark 2 - 5 port muzzle brake, name in gold on breech block, power adjuster in cocking button;
Mark 3 - 3 port muzzle brake, longer breech block forward of loading port.
I have seen variations where a Mk1 has a breech block with the name in gold so there are evidently some transitions but the above is largely correct.
People who have been there focus on the fundamentals. People who sit at keyboards all day focus on the trivial and inane.