For many years I have observed a peculiar use of the word 'off' in electronic parts lists, where one might write
3 off left handed toggle widget
meaning three of them are required.
I had always assumed this use, which is not mentioned in my dictionary, was local to the electronics industry. Today, however, I noticed "Enclosed Turnstiles (20 off)" in the dome auction brochure.
Does anyone know where this use comes from? Is it a corruption of 'of'?

Phil