Since ammo was seldom used directly from the box, but instead loaded into stripper clips (chargers), mags or whatever, it really makes no difference how many a box contained. Of course if your theory is that they were being loaded into the 5-round stripper clips, I can see where you might throw in a few extra for the ones that soldiers dropped into the mud fumbling in the trenches (just kidding). Since the previously used military round (577/450) was packed in packs of either 5 or 10, I can't see where the Brits were hooked on the number 12. However I've got .303 loaded by a minimum of 8 countries and they all are magically 48 round boxes.....except for the 20-round Winchester white boxes of the 1980s supplied as aid to the Afghans during the Sovieticon incursion.