Calimero, I have a copy of Histories et Collections "Le Soldat Francais 1940". Its in French but beggars couldn't be choosers. In the section on helmets there is a picture of the disc with the cuirass, plus a helmet and the latters RF ( Republique Francaise?) . My noddy French plus Google translate says this was the insignia of Engineers, military railway and military "telegraphy" ( Communications?). A bit niche but that's what the book says and its normally pretty correct.