The information you need is in Pirkle's book on the 66, 73 and 76. Very detailed about sling swivels since if not factory installed, collectors consider them a major deduction. Winchester had 2 basic types: a stud with hook and eye, and a nondetachable swivel. Both types are a stud in the fore end cap that is riveted on the inside. Through WW2 Winchester sold the front stud, rear stock screw, hook style swivels and a sling for $1.40. You could put it on your self. The problem is that without Winchester factory tooling its hard to center the hole. The factory 76 rear swivel is a 2 screw plate 1 3/8" long and 1/2" wide mounted 4" ahead of the toe. it was inlet into the wood. The sling loop is attached. The front was the fixed loop and stud riveted through the nose cap . I personally would prefer the hook and eye because it works pretty much like modern QD swivels. I don't know if any of that type were factory installed. Jeff's outfitters used to sell a reproduction of the hooks. Dixie still list them. They also show up on ebay. If you don't have access to the book I can scan the pages and send them to you.
John