Ray - when I first encountered round with these primers (in fact, likely the ones I showed) I asked our Remington Rep to find out about it. I can’t say this is gospel. Sometimes the factories or their reps tell you anything just to get you off their backs. I have found they don’t like questions about their ammunition, for the most part. Regardless, I was told that the “O” was put on primers sent to other commercial houses (primarily commercial reloaders) so that should there be any trouble with the cartridges themselves, Remington could know that they were not a Remington-loaded product. For example, all of my .30 Luger blanks are, coincidentally, with REM-UMC headstamp, but they were not loaded as blanks by Remington. I don’t recall ever seeing these 'O" primers on primers offered in gun shops for sale to individual reloaders, but then I don’t know the time frame that Remington made these marked primers. I did not start in the gun industry until about late 1963.
Regarding the meaning of the “0” itself, I got no answer other than that it just was a symbol that was easily applied to the primer cup and had no special meaning as a letter or digit, but rather only signified what we were talking about, that it was not a primer Remington themselves put into any factory-loaded ammunition. Again, can’t vouch for the accuracy of that, but it is well known that while most things on cartridges represent some particular thing, the actual symbol used isn’t always the initial for any particular word or term. The CCI “A” primers are a good example of that. the “A” by itself, unlike the “B” for Benchrest on their primers, has no meaning standing alone.
I am sure that during the tests for the NATO cartridge, there was plenty of input from Civilian industry. Few of these cartridges originate solely in military arsenals. I think you are probably right about testing different primers, anyway. I would guess the domed primer, which I have seen on rounds, is Winchester as well. There was a question on the Forum recently about FA helmet test .45 rounds from WWI. Although Remington primed and headstamped cases were used, those rounds were actually loaded at Frankford Arsenal. That brings up the question why they didn’t use their own brass. Again, all of these arsenals get assistance from private industry, I’m sure in other fields of militaqry manufacture, not just ammunition.