I have grouped together the Mk2 and Mk3 series helmets in this overview as they not only are very similar in design, but also due to both Mk's being in service together, some modifications were common to both Mk's.
The Mk2 flying helmet was the first one-piece protective helmet to be issued in the RAF or RN.
It's predecessor, the Mk1A was a two-piece assembly consisting of an inner communications helmet (F-Type or G-Type), and separate outer shell.
The Mk2 helmet featured some new concepts, never seen before in British helmets:
Being a one-piece helmet, it is quicker to don / doff than the previous two-piece helmets, so good for QRA, but the disadvantages are: it's harder for the squippers to maintain as access to the internals is limited; you don't have the choice to remove the outer shell on long flights (think Vulcan crew).
I have a theory (my own personal reasoning) as to why the single rear sound transducer was used in preference to individual speakers in the ear capsules. I believe the design intent was to reduce the over-all width of the helmet, leading to a more compact design. This may have been beneficial to aircraft entry / egress, reducing ejection wind resistance, gaining parachute line clearance, etc... If anyone from Helmets Ltd is reading this, please let me know if this is correct or not.
The Mk2 only had a single visor, so the automatic lowering mechanism was critical for aircraft fitted with ejection seats to avoid wind blast to the aircrew's eyes.
The Mk3 series of helmets used the same basic shell, single transducer headset, and liner of the Mk2 and Mk2A helmets. Later the Mk3 helmets would receive a new type of liner to suit the AR-5. Variants of the Mk3 series allowed for different visor and oxygen mask configurations to suit different flying roles.