I got on to this as I was looking for something completely different...

On engineering drawings you'll always see "manufacture 5 off", and never "5 of", for a quantity of 5 items. I think this has to do with "5 items off the drawing, or bill of materials", which leads me to believe that Peter is the only person here who got it right.

Anyway, suppose you shouldn't trust an Afrikaans speaking engineer to advise on the subtle aspects of English.