I would think it's probably Box Magazine, Bren Gun, Carriers, Web, that is, a 'Web Carrier' for the 'Box Magazines' used with the 'Bren gun', as opposed to a Box for magazines used with a Bren Gun in a Carrier made of web (which doesn't make any sense). I could be wrong, though, as army nomenclature often doesn't make much sense.
Could be absolutely anything, from military re-use to use as a schoolbag or any other thing you could imagine. I am not aware of any specific military modification to it. It's not a fake. I used to have one that was full of Box Magazines when I bought it, although I don't have it now (still have a Bren Gun though).
Rob
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