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Not sure how "How Much do you Type" is off-topic, but if so then this is even more so - though it comes up often with old typewriter ribbons: What the heck is "New Old Stock", and how does it differ from "Old Stock"?
It doesn't differ at all except for the extra hoorah word and the resulting oxymoronic redundancy! Old stock is not new, obviously, so what is really meant by "new" here is unused. But Old Stock already connotes unused! If it had been used then it would not be "stock" at all - it would be "used". The extra word is oxymoronic and redundant simultaneously - quite a linguistic accomplishment I think you will agree. It is also a monument to word padding - something the presence of a few actual WRITERS on this group has made me conscious of in my own feeble term paper equivalent efforts, and which I strive to eradicate. Just can't stand cant.
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Now you've got me to thinking. I wonder if "Old Stock" means ribbons that have been used, but still have a bit of service life in them, and "New Old Stock" means unused ribbons, but discontinued makes and/or brands. For the former, "Used Stock" would be more accurate, I would think. But then who sells used ribbons? I'd sooner give one away if the person needs one that badly.
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In the wacky world of wristwatch collecting, "new old stock" generally refers to a watch that is no longer in production, but was never sold. Basically, it sat on a shelf or in a store-room for decades.
This term has been bandied about quite a lot in recent years.
As TypewriterKing has stated, it would seem that new old stock ribbons are just ribbons that were never sold back in the day when typewriters were found in every office.