Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
The first, third and fourth all have the same chin-straps, the gunner's is certainly different.From left to right, the the first three guys are wearing the then new '44 Pattern' helmet, still in use up to the early 1980s !
The chap in the background appears to have the earlier, 'Brodie' style helmet, in use in various forms from around 1915 until 1944 and slightly beyond.
EDIT:- Just had another look, and it appears that the gunner is wearing a late-style Brodie helmet, the design being sort of 'in-between' the '44 Pattern' and the standard Brodie, same as the guy in the background.
Yep, I tried using one of those when I was involved as a Partner in a company using Chieftain, Scorpion, Abbot and '432. Bl**dy awful, so I used a RAF Mk4 helmet or, as you did, headphones and boom mic, over a 'crap hat'.