Are you able to get an exact match to a known WW2 helmet or other decal including size and colour? While it could well be a fake or a partial post war fake/creation there is always a possibility that it's a 1off war-time "special". From the pictures the decal looks like a transfer and it's not been painted on. If the decal is of a size that wasn't readily available during WW2 then it would appear to me to be a post war add on. I wouldn't have thought that it would be too difficult to check on the decal which you have for it's WW2 authenticity.