The forward stud could be either soldered or screwed to the underside of the rib. Which is better depends on owner's preference but maybe also on the details of the rib geometry and construction. If you send it in, Mark at NECG can tell you what options would work for your guns and their respective pros and cons.

I'd hesitate to put swivels on a very fine old British double, but pretty much any Continental gun I decide to keep will probably get them at some point if they're not already mounted. I've tried a couple different slip-on versions, but they all seem to slip off on their own. I may give the Accuriser slip-on sling Gunut linked to a try. Good Luck!
Bill