World War II postal acronyms
World War II postal acronyms were first used to convey messages between servicemen and their sweethearts back home. They were usually written on the back of the envelope.
The acronyms, possibly including some more recent additions, are:
Terry Pratchett parodied this briefly in his Discworld novel Going Postal. His acronyms included L.A.N.C.R.E. and K.L.A.T.C.H. (Lancre and Klatch being two locations in the Discworld), although these were never officially expanded.