Thankyou VERY much indeed!

I worked in an Engineering Factory in the 1970s. We used the term "5 0ff" (or whatever number), very much in the way prpr suggests, to refer to the movements of a quantity out of stock, often literally "off " the relevant shelf and than "off " the inventory record. A "one-off" was something manufactured for a single, specific and often unique order that would not normally be held in stock. Of course that doesn't mean we were correct. Some interesting comment here:
I don't know where you live BH but Bristol, Cardiff and Newport public library members can access the OED on-line version of the 20 volume OED. It also includes quarterly revised entries for the 3rd edition.

Go to and click on sign-in. Two sign-in options are then displayed. Select the one which asks for your library card number!
That will do nicely.