Joe,
The best source of general info on these helmets is L. Baer's "History of the German Helmet 1916-1945, i belive that it is curently OOP but copies can be found available for sale on the internet. Kelly Hicks well written works also present some images of parade helmets of SS lineage.

It is believed that Vulkanfiber (form of fiberglass) constructed helmets were considered to be a replacement helmet over the steel WW1 designs around the 1932-33 period and actually saw limited use with short-lived prototypes fielded for trials. These helmets were ultimately rejected in favor of the M35.

The TR period involved a number of these helmets being retained and refurbished as light-weight helmets intended for non-combat parade/function roles. Additionally newly manufactured helmets of similar process technique using vulkanfiber were made available as private purchase items for the same parade/non-combat roles. The firm Erel was quite involved in manufacturing and supplying these items.

These fiber helmets as well as helmets of lightweight metal construction were supplied to several oganizations during the period, SS included. It is also possible that a protype of the M34 civilian metal helmet was originally a vulkanfiber prototype helmet intended for the fire service.

These fiber helmets continued to be manufactured post war and were extensively used within the fire service of the DDR through I believe the 1960's and possibly as late as the 70's. Erel seems to have been a major source, and the helmet went from a parade/ceremonial role in the NSDAP era to that of a service role as a duty helmet affording protective headgear to fire personnel. Construction method was very similar to the original NSDAP versions. But there are differences in the shapes of the shell and with the components (vents), often being very subtle. Typically the suspension is a co**** grained, stiff leather "M31-style" fingered design, often utilizing a four point "Y" harness, rather than the very high quality finished leather or silk and leather products found in a TR period parade helmet.

Like everything else, these post-war manufactured helmets are commonly "dressed up" to simulate the earlier TR parade helmets. Another reference covering these post-war helmets was again authored by L. Baer, "Vom Metallhelm zum Kunststoffhelm", text is in German, but it is a very good reference.

I hope that this has helped,
Regards,
Don