It's certainly in VERY good condition for a 1975. I suspect someone held on to it for the same reason you are asking, and when they realized it was not, they spent it. Even an impaired proof, will have incredible detail and more luster than normal. The 1975 no S is not something you would find in circulation, as someone would have had to break it out of mint packaging, and spent it. Which is possible but highly unlikely to find a needle in a haystack like that. Yes these million in 1 finds do happen, but the odds are it's always someone else.
@David Handlin Here is a 75S for reference. Notice the darker mirrored surfaces and the frosty devices. Here is a closer look at the surfaces. There are no radial die striations. Square evenly centered rims. This is a S/S RPM
