grammar - A number off or a number of?

I am reading some technical documents and there is a list of items that make up the product. Throughout the document where there are multiple items, they are listed as 2 off, 3 off and so on.

For example it would be similar to this:

My product comprising of:

• 2 off card 1



• 1 off card 2


• 1 off transit case

Is this correct? I thought it should be 2 of, 3 of and so on. I would like to know if the usage is correct and if possible why off is used in this context.

Answer

BEN ZIMMER, On Language The Origins of ‘One-Off’:
The New York Times (Magazine) July 2, 2010

… this British usage of off typically appears with a number to indicate a quantity of items produced in some manufacturing process. The Oxford English Dictionary, … takes this back to a 1934 quotation from the Proceedings of the Institute of British Foundrymen: "A splendid one-off pattern can be swept up in a very little time." Other numbers can fit the bill, as in the O.E.D.'s 1973 example of an advertisement for "Kienzle printers, 6 off, surplus to manufacturing requirements.

"Though some Americans might still find one-off annoyingly British-sounding, there's no need to fear it as a solecism. It's a well-manufactured word, even if it feels a little off-kilter. [emphasis mine]

I think that is quite self-explanatory.