No, I don't think so. Neither the stitching nor the buckle itself look like a helmet strap.
I'm inclined to agree with Terry, some other kind of equipment strap I think.
Thanks so far. Just compared it to my helmets. Am afraid it is different. I have scanning the place for years. Many military relics from the 1600s on. But from ww2 only british relics so far. There used to be a workcamp nearby in ww2 guarded by the ss. No relics from them so far apart from 9mm bullets and casings.. o well.. beter luck next time
keep on looking.
Pity about the buckle, but it sounds like you have an interesting playground to explore