My impression: not ex-arsenal, and not Bubba either, but the work of a local unit ("base workshop") to keep a rifle in action, lacking the original magazine, which was presumably damaged - they don't just fall out. Collectors often forget that the armorer's job was to keep the guns shooting, not to produce perfectly matching equipment in so-called "correct original condition" for said collectors to enthuse over 75 years later!
To use the well-worn phrase, that magazine is "part of its history". To replace it, for instance, in the misguided attempt to make it more "original" or "correct" would, in fact, be a falsification. I am not suggesting that that is what you are intending to do, merely taking the opportunity to point out what is, alas, all too frequently done.