As far as acronyms go, the vast majority of genuine ones post-date WWI. Anything that claims an older origin is almost certainly untrue. WWI military terminology made commonplace such phrases as Ack-emma, Pip-emma, Toc-H, Ack-ack, etc (all of which are from the WWI signallers phonetic alphabet).

If you see such claims as "For Unlawful Carnal Knowledge", "Ship High In Transit", and so on, your best assumption is that someone is either trying to con you, or that they've been taken in by a plausible sounding story!

Blag, by the way, is in my OED, with the suggestion that the origin may be from the French blaguer "tell lies", at least in the sense of using the gift of the gab, if not in the sense of violent robbery.