Message 94, 2 December 2003
Bluelight and other interested members:
In addition to the 3 known examples you mentioned, I know of two more in private collections, for a total of 5.  Additionally, I believe there is at least one at the Naval Aviation Museum which I spotted on my last trip.
Best, Bloodhound

Message 92, 1 December 2003
Hi all,
I made a mistake in my last posting on this topic when I claimed that both Figmo and Bloodhound aired the view that their mystery helmets were early HGU-33s. Figmo actually said: "the helmet is transitional ie this ain't no APH-6 and not a PRK-37/P. I think this helmet is a prototype developed by Sierra Engineering" (cut and paste from two messages). Bloodhound bought his yellow helmet from Paul Taylor and both of them identified the shell as an early HGU-33 and not an APH-6.
The helmet is not necessarily a prototype since Sierra offered it for sale in their 1973 catalogue (at least three examples survive; Bloodhound's, Figmo's, and allegedly Pete Olsen also has one). If Figmo, Bloodhound and Paul Taylor all are right the helmet shell should be a precursor to the Gentex DH-151 shell which was later standardised as PRK-37/P and used for HGU-33/P, HGU-34/P and a whole host of other US Navy helmet configurations. Thus the question about connections between Gentex, Protection and Sierra remains.
Cheers,
Bluelight

Message 66, 27 November 2003
Hi Bloodhound,
Funny that you should bring up this topic. Lately I have been trying to find more information about the major industrial players in US flying helmet development and production, Gentex, Protection Inc, Sierra, and Scott, and MSA is also three somewhere. I am specifically interested in the buy-outs and takeovers including timelines because it also defines transfer of technology and copyright.
The reason for my interest right now is that I am still struggling with Figmo's blue "mystery" helmet and your yellow VF-142 helmet, both of which have the modified APH-6 type oxygen mask receivers with a hinged centrebox. When we last discussed these helmets you and Figmo both aired the opinion that they were early HGU-33 helmets rather than APH-6 derivatives. My problem is that the DH-151 helmet shell, which was later standardised as PRK-37/P and used for HGU-33/34 and other US Navy helmets, appears to be a Gentex design. Figmo's and your helmets, however, were made by Sierra Engineering as evidenced by the Sierra Engineering 1973 catalogue. I have included a number of pictures that illustrate my points.
As we discussed earlier there are two versions of the bayonet receivers, the ones that are on your helmet and the ones that you have in the Sierra p/n 345-315 retrofit kit. As you can see from the pictures your and Figmo's helmets are identical to the one shown in the Sierra catalogue. The probability of your helmets being Gentex DH-151 shells modifed with a Sierra retrofit kit is therefore very low.
I have a theory that the DH-151 might originally be a Protection Inc design that Gentex took over when they bought PI, but I still need to find a link between Protection Inc and Sierra before I would find it likely that your helmets have shells made to the same specs as the DH-151.
Cheers, Bluelight

Dark blue "mystery" helmet posted by Figmo and yellow helmet bought by Bloodhound on eBay

Custom fit helmet from 1973 Sierra catalogue and Sierra retention set p/n 345-315
bought by Bloodhound on eBay

Sierra bayonet p/n 345-340 from 1973 Sierra catalogue and DocBoink's 345-340 bayo
inserted into the receivers on Figmo's helmet