I use clinch tacks on a regular basis building saddles & harness repair.I use them to hold 2 pcs of leather together that doesn't need to be glued before stitching.In the harness making industry contact cement doesn't stick to harness leather because the leather has waxes & oil infused into it at the tannery so clinch tacks hold the pieces together sometimes they are driven all the way in making the clinch & become part of the fastening system .Temporary clinch tacks I will sometimes put in the stitch groove & pull them out as i sew up to them the thread covers the hole.There are tube rivets that are made solid except a portion at the end which is hollow that will roll over & capture the material at the bottom eg tin bound stirrups.I'm thinking this might be in your case these rivets are made from steel,brass,copper,aluminum so on & so forth.I don't know a supplier but I do know Weaver uses them on their tin bound stirrups as I've used them on lower end saddles.Hope this helps you out not examining the sheath its only a guessing game.How I use the clinch tacks is I put a piece of steel under the work & that will turn back the end forming the clinch.
Cheers, Ken