Hey Mark,
You are right, the British mask bag has a leather tab that engages a metal stud to shorten the strap for the ready position, where the American (OK, US) bag strap has a bras claw that hooks into a buckel. You are also right that the French and Belgian masks have and exhale valve, and it is true that the filters are not totally like the German filters, but they are about the same size and are changed in the same manner. I have seen a period photo of a Belgian soldier with and unaltered German Ledermaske, there are some people who believe that the leather Belgian masks that we encounter are actually post war, but I do not know for sure.
As far as the PH helmet goes, do not waste time sending a photo, just send the helmet, I would love to have an example, as they are very few and far between.
Best wihses
Gus