Many Mk2/2A flying helmets were converted to Mk3C standard by installing a new visor kit in place of the original Mk2/2A helmet visor system. This is the reason that Mk2/2A helmets are so scarce these days.
The give-away clues of a conversion helmet are the screw holes left in the shell from the Mk2A helmet visor assembly, and 4 holes at the rear for the original aluminium data plate (if one was fitted).
A lot of the converted helmets will have the early style "flatter" profile visors, using two screws on the outer tinted arms, and three on the inner clear visor arms.
This extract from a modification record highlights two versions of modification used to modify a Mk2/2A helmet to a Mk3C.
Quote:
Mod No Brief details Class C5/74 Introduces a double visor system in lieu of the existing single visor. Embodiment of this modification re-identifies the helmet as a Mk3C variant which is described in AP 108F-0203-12 SOO C10/76
(Mk 2A only)Supersedes Mod C5/74
Introduces double visors cut from existing polycarbonate injection moulded visor designed for motor cycle use by Stadium LtdB/SOO