IIRC, the down tube on most vintage steel tube sets had the largest diameter, so I don't believe that could be a re-purposed seat lug. But then again, it's French, so...

I think the guy who sold the NOS with bag (I actually believe it's the bike's travel bag) is closer to the correct price. One in this condition, with non-original parts, maybe a couple hundred for the funky-factor. Of course, a fan of one of these (there were people asking about availability in both the ad and BF thread you posted) might certainly pay more.

I let the dude know what he had. His response was a short and sweet "thanks". Maybe he'll change his ad copy, maybe he won't. Seems he is holding out against hope for a big payday - must be worth a lot because it's chrome, you know?

DD