Regardless of what exactly a WWII U.S. military headstamp is supposed to say, military ammo is generally not stamped with the caliber, regardless of its country of origin or use. Most military forces use only a handful of standardized calibers that are easy to tell apart, and for obvious reasons. An infantryman in a dark and muddy foxhole with mortars landing nearby and machine-gun rounds crackling overhead should not have to read a headstamp to load his rifle.
OTOH it's important for a military force to be able to quickly pull a faulty batch of ammo from the field- ammo that soldiers have already loaded into magazines, ammo belts, bandoliers, or whatever, so they can't rely on what was printed on the box. ("Uhhh, it was a big green metal can. Just like all the rest of them.")
Hence, most military ammo is stamped with alphanumeric gobbledygook denoting the date and location of production.