Original slings that I have seen incorporated a snap hook on the leather sling. The snap hook goes through the sling eyes on the barrel and butt of the rifle.

Here's a photo of the original sling and attachment with my Woodward .450/400 3 1/4" double rifle:

There is a very similar arrangement with an Alex. Henry double .577BPE I recently acquired.

I hope this helps with your question.

Curl