Not sure if this applies in other careers but...
being in the army means there are LOTS of lists (whats in this box, on this shelf etc etc) and for some reason if there is more than one of any item it is listed as "2 off" "3 off" etc.
its 2 OF , 3 OF ffs...

and dont bet me started on the middle management i work with who are totally incapable of stringing together a simple paragraph indicating work they have done or want doing... drives me up the wall.

_/length
( Crack <--- Lurker..... came out of hiding on, Thu 15 Apr 2010, 13:14, 3 replies)

You're not, er ...
You're not "sounding off" about this, are you?
( Je suis un vagabond is an unfunny, up your own arse middle class knob, Thu 15 Apr 2010, 13:20, closed)

I thing it's a warehousing term
and is short for "2 off the shelf".

I work in electronics manufacturing with bills of material and this is pretty commonplace. When I started four years ago I challenged this in the same way you have, to be given the same answer I just have.
( shinyshinyscalp less a man, more a way of life, Thu 15 Apr 2010, 13:23, closed)

I've never heard quantities
... expressed as "one of"; but all of the places that I've worked in manufacturing use the "one off" form. It appears to be the universal idiom rather than an error, and so I suppose it is the correct usage.
(snopesthetruth, Thu 15 Apr 2010, 13:26, closed)