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Sierra Corp and Gentex Inc sold single and dual visor HGU-26/P helmets are dated late 1974.  Single visor helmets based on the HGU-22/P shell and manufactured after late 1974 are therefore single visor HGU-26/P helmets, not HGU-2A/P. 
In the late 1970s it had become commonplace to use custom fit liners. Most customers had no use for the styrene energy absorbing liner.

As a consequence Gentex and Scott started to field the so-called Quarter Helmet Assembly. It is comprised of a bare HGU-22/P helmet shell with chin and nape straps and either single or dual visor assembly completely installed.

The HGU-2/P, HGU-2A/P and HGU-26/P helmets have been modified and upgraded to meet every user's whims over the years. The most common modifications are custom fit liners instead of the standard fitting pads and the single visor cover gullwing cut to improve the pilot's upward visibility. Both these features are included in the helmet prepared by Protection Inc. for the USAF Thunderbirds display team.

A modification kit known as PRU-36/P visor housing kit is to provide the same improvement in upward visibility as the gullwing cut on the single visor housing.  It can be supplied with tinted , gradient, clear or anti laser visors .

The customers would then install comfort liners, oxygen mask retention receivers and communications to suit their operational needs. The helmet circulated with US Armed Forces, NATO, FMS etc.

This helmet in single , dual or anti laser visor configuration with or without MBU-5/P or MBU-12/P Oxygen Masks is still available from Aeromiltec Gmbh on a minimum quantity order of 20 units .

PRU-36/P  Kit