Jim, Henry the V helmet has the same piece on the back of it. Pics here
http://www.myarmoury.com/talk/viewtopic.12798.html . These helmets are not " light ", Henry's I believe is something like 16-17 pounds so that bracket at the back being a belt loop seems unlikely for at least that example. In tournaments, it was not uncommon to secure a re-enfocing plate on the left side of the helmet. I have wondered if these brackets may have been for securing this tournament re-enforcing piece.