The Mk 2A was the fast jet bone dome, with the visor inside a protective cover, and a G lock arm which brought the visor down in the event of an ejection. The Mk 3 was (generally) the helicopter version, with the visor on a central track, as per the Mk 1.
Both Mk 2 & 3 versions were fed from a central speaker which piped audio to the earpieces via a plastic tube. Next to useless when the earseals broke, and the gel dripped into the tube & blocked audio, or leaked into your ear, or down your neck & under the goon suit neck seal
The Shackleton crews also wore the Mk 1 bone domes, preferring the cloth headset for their long sorties. One trip out of Ballykelly, I turned up with the Mk 3, they were good enough to search out an old Mk 1 for me: the sort I used to use riding my motorscooter back when....