To truly understand the significance of the swjXE code change, you must first understand the code system that the Germans used. It's really tough to explain why it's important if you don't collect the 98k rifles. This is why many of the swjXE rifles in existence aren't noticed for being collectible, it's just some letters to the unknowing. Add to that on this example there are no nazi eagles, some may think it's a postwar rifle (I've heard people say they thought they were French because of the svwMB rifles).
Anyway, not sure where you could start and really understand it. I guess start searching for "Steyr" and "bnz45", look in the picture reference thread and look at all of the bnz made rifle threads. Even then, if you don't really know the context of the whole mess it's probably confusing.
And yes, US troops captured the factory making these guns. There are more bnz45 rifles out there than you can shake a stick at. I have more 45 dated Steyr production reported than any maker, but by serial number they made less than any other factory. The reason is, a large portion of the bnz45 made guns were snatched up by GI's and sent home. I would dare say bnz45 (which is the same maker as swjXE) is the most common 45 date rifle by far.