A piece of information to be added: an other marking, VAV, that was crossed out, can be seen next to KB. VAV stands for Versuchs-Abteilung des Militär-Verkehrswesen (Experimental Dept for Military Transportation). Thank you, Philippe, for the interesting discussion we had a few years ago on the topic. This helmet moved from VAV to KB, which sounds logical. The nomenclature of those minuscule technical units is not easily deciphered though!