The study of helmets markings is a field unto itself. Are there anymore markings such as beneath the bale or on the underside of the liner etc?
I can't be of much help but I'm inclined to believe the marking you show is either a steel batch or heat treat code.
There are militaria forums and great war forums out there where you might appeal to a much more familiar audience. Give this post some time. There are contributors with a wide range of interests and perhaps a few of them could shorten your hunt for an answer.