I have not seen this before. Do you think you could ask the owner for a copy of the picture to put on the list? Here is what I think it stands for. I'm not positive and I could not read the French answer to the question posed on eBay.
Bekleidungs Instandsetzung Handwerkerabteilung
that might not be true as there is another kind of BJA mark that is more standard already located. But it makes sense to me that this stamping postdated the ones on the left-hand side of the neck guard.