At Avionics in '70, I logged in and repaired all the helmets- logged on DA 2407, along with Nomenclature, NSN and/or FSN, Type, Line Number,
Quantity, Color, Manufacturer, etc., etc, ad nauseum...

ALL I ever saw were SPH-4, APH-5 and -5a. (as if I knew the difference)... they were identifed from the labelling. If label was missing, it was not logged on 2407, just repaired on the spot and returned to the crewmember. Lotsa brain buckets-
most only needed replacement or repair of mike cords, or reconnection of loose wiring, or mike booms, due to "bungee-jumping" crewmembers...

As to the tailrotor, not being an A&P-type, I'd say that was the US M-1 go-'round fast- break-easy
Model....

Lump
Internet bill is paid.