Weve been over the retractable sling here, quite a few times. When I was importing Darne guns, that sling was an option. I sold few of them, they were expensive, and while they are a bit gee wiz, they arent really a very good sling. The width of the strap is narrow, supporting a 7 lb. gun on a 1/2 strap isnt that comfortable. The mechanism in the butt adds weight and mechanics right where you dont want them. The sling was not a Manufrance product, and they appear on all makes of French guns.
To the guys who wanted a sling, and, that wasnt very many, I always told them to go with the bretelle Darne. I use a sling on a shotgun, and there really isnt anything that compares. The front loop is free to move up and down the tubes, eliminating most of the complaints about the sling getting hung up in brush. The sling is easily removed when you dont need it, and easily put back on when you do. I just leave it in place, on my own guns, usually.
Given a choice between sling swivels and a regular sling, or a retractable sling, Id take the former. It is just a better sling, all the way around. If you are going to have a sling, have a good sling.
The retractable sling is really only for very intermittent use. I guess if I was restocking an old gun that had a functional retractable sling in place, Id at least consider putting it in the new wood.
Best,
Ted