Rivet head dating
Also the liner bolts have been given a plastic coating.
The earliest models, made by Thetford Moulded Products, have a label that reads "HELMET, PARACHUTISTS, Light Weight", but after production was taken over my NPAerospace by the late 1980s this became more simply 'HELMET, PARACHUTE' There appear to be three variants - the earliest version has a smooth surface to the shell, and usually has bare-metal liner bolts.
The liners do wear out of course and if you exchange yours with the RQMS, it will eventually find its way back to the factory for refurbishment and then re-issue.
When I visited, there was a great big bin of them waiting to be worked on.
This shell is simply fibre-glass and offers no ballistic protection.
The shape of the shell remains the same with later production runs, but the surface is textured.
It is just amazingly cheap and nasty in every sense.