Well, for saying nice things about the engraving thread, I feel compelled to take on your challenge.
Shooting sports and hunting methods varied greatly from country to country in the mid- to late-19th century, as they do today. In the 1860s in Britain, game was primarily walked up, and the driven shoot was only just starting to develop. British shooters did not use slings, and no guns had sling swivels or any mode of attachment for them. I am loathed to give such a sweeping statement, as with Victorian gunmaking, anything is possible. But I have yet to encounter an early breech-loader, pinfire or centrefire, made by or for the British market that has sling swivels, and I’ll maintain this opinion until proven otherwise.
Over on the continent, things were very different. French, Belgian, German, and Austrian shooters also walked up their game birds but often employed slings that, to my knowledge, were made of leather. Thus continental guns from this period are usually fitted with sling swivels. The sling attachment is non-detachable as far as the gun is concerned, and the leather sling would pass through the iron loop and have a roller-fitted buckle of the type you see on gun cases. There is also a curious plug-type attachment on some butts, which I’m guessing would fit into a slit cut into the leather. Perhaps such a sling is easier to remove or adjust in length – I don’t know.
Here is a vintage continental sling demonstrating the buckles. I don’t know the age, but as the leather is in good shape, I’m guessing the 20th century.
Here are the two styles of attachment mentioned above. The first is on a 12-bore Lefaucheux-type pinfire sporting gun by Châlet, Père et Fils of St. Étienne, France, serial number 10, dating from around 1860. It only has part of the swivel base, and the actual belt loops are missing. Presumably, a previous owner did not use a sling and discarded them.
The other attachment style is on a 16-bore Colleye System pinfire sporting gun signed August Gottlieb Schüler of Suhl, Germany, but made by Maximilien Nicolas Colleye of Liège, Belgium. It too dates from around 1860. The barrels still have the full swivel, and the butt has the curiously rounded plug.
Let’s see what else turns up. With the beautiful work done on drillings, there must be some high-quality swivels out there.