I was about to jump in with both feet siting the fictional character in Star Trek: Voyager: 7 of 9 as being definitive proof, but knowing the Forum thought I'd better check. The dictionary on my Mac states: so looks like both are acceptable.
I was sitting at my Mac when composing the original email, didn’t know it could do that.

I would be interested to hear more on the Star Trek suggestion, it means nothing to me.
I certainly agree with you that 'off' is common (even normal) usage. But perhaps, more normal syntax is:
widgets: 5 off
rather than
5 off widgets?
PS: I certainly wouldn't say "5 of widgets". That makes no sense; the 'of' is redundant.
Yes you’re right, transposing the columns is probably better syntax. Maybe that is what I was taught, but only partly remembered.
Never heard it being used that way. After all, its "2 of a kind" not "2 off a kind".
Surely you'd say "5 widgets", as like you say any more words are redundant.
In the context I formed the habit as a youth, I’d probably have been ordering electronic components, with a description that is quite long winded and already includes numbers. For example,
4 off 27k 1/4W resistor.
The ‘off’ is a useful separator. But I’m now thinking it is also likely that I would actually have used Eric’s syntax. I have no evidence however, and no old copies of communications. In those days, such letters were simply written in my neatest teenage handwriting, keeping at most just a messy carbon copy that was soon discarded.
Other comments still welcome, but I currently feel no need to back down in my domestic dispute.

Ps: It did occur to me to compare it to the phrase describing a unique event as a “one-off”?