Hello oldman
My first job at my first unit RAF Cranwell was modifing Mk 1 helmets to Mk 1a standard fitting the track and visor mechanism which involved drilling 4 holes and securing the track with self-tapping screws coated with a very sticky varnish to help prevent the screws from working loose.
Later helmets had metal nuts moulded into the helmet shell.The Mk 1 helmet did not have an adjustable harness. It came in 14 different sizes and fitting was by trial and error. The helmet being a snug fit over the cloth 'G' helmet if it was too tight the wearer could end with an excruciating headache.
Well thats what aircrew useally told me. Perhaps that was the main reason rear crew members prefered not to wear them after take off.
When tall aircrew flew in Chipmunk aircraft wearing the Mk 1a helmet the visor operating knob could touch the canopy!! there was then a modification to saw 1/2 inch off.