Stan,
When I was in France, the Darne works occupied a small corner of the original factory. The factory is about two city blocks long, and has been cut up into condomiums, lofts and studios.
There is a sculptured arch, with the word "Darne" cast into it, over a concrete staircase that leads one from street level on Rue de la Convention, to the second floor on the back side of the building.
It would be hard to confuse it with an old schoolhouse.
This is a photo of just the arch, from the reverse of a French language Darne catalog. My catalogs had The Drumming Stump bird image, that I lifted from the DGJ, with permission from Joanna Coté:

I offered 410s and sold 28s when I was importing them, not many of either, but, nobody ever told me no. I also own a 28, and they turn up for sale here and there. If your friend was told no, I wonder if it was because they realized the end was near, which, would put the date in the late 1970s, 1979 to be specific.
Not many of them out there, but, here is a 28, that my kid has his eye on:

RE Darne .410s-if you like the original Winchester model 21 .410, you will love a Darne .410.
Best,
Ted