They where developed in the 3.Reich as M45 / Type BII (prototype was registered as patent on December 7, 1943), but only testet in combat by the Wehrmacht in small numbers (picture from September 1944, Westfront):
source: https://archive.org/details/DieDeutscheWochenschauNr.733 (Minute 5:48)
In GDR M45 was then improved to M56 NVA version.
-> https://de.wikipedia.org/wiki/M45_(Stahlhelm)
Supposedly, before the introduction of Kevlar helmets, they had the the best ballistic protection and earshot range.
Would be cool if this could be simulated in CMCW. (like very small percentage increase for shrapnel protection and command audibility range).