OKMNX

Same old story... found a mysterious envelope amongst my dad's papers.

The mystery deepened.

This card accompanied the letter.

And for good measure this little ad was included.

Which was but a taste of the full treatment afforded other papers.

Oh, and if you remain puzzled by the acronym... read this aloud
F.U.N.E.X.
S.V.F.X.
F.U.N.E.M.
S.V.F.M.
O.K.M.N.X.