Gene,
Does your gun have sling swivels? Jeff's Outfitters has 3/4" and 1" slings on the shelf. I've heard good reports about the quality.
I think I remember you saying your gun was a12 gauge R10. If it has the little oval fitting in the stock for the Bretelle Darne, it is a rare bird-few people ordered that sling for that grade of gun. Geoffroy would NOT have a spare or inventory on the shelf. But, he goes to France in the spring, and could likely bring one back. As I warned, he may have had enough of this, as sources for the components are drying up, along with the gun industry in St. Etienne. I can't really imagine what it would take to get a bretelle automatique on a gun that was here, in the US at this time, but, I'm guessing close to impossible. Make no mistake, St. Etienne as a gunmaking center is in decline.
The recent immigrants aren't big on hunting the local pigs, you could say.
The easiest would be the loop sling, truly uneffective while hunting, but, useful for carrying. The next would be having sling swivels fitted, better, in my opinion, but, not a bretelle Darne.
Good luck in whatever you decide to do.

Best,
Ted