I finally made the time to look through my parts drawers and found my last remaining set. These are the ones made by the American company which is now making some of Uncle Mike's discontinued items.

Info is:

Grovtec US, Inc

GTSW-30 Fits 1" Slings
GT locking Swivel Set

For Side-By-Side
Double Barrel Shotguns.

I'm pretty sure that these are available from Brownell's.