Thanks you for the picture and description, Wicke. I have made some sheaths over the years, and remade one or two on occasion, so I thought the process might be something like this. I didn't know if you left extra leather beyond the welt to grag in a vice, or how you got that straight seam down the middle. Now it makes sense. Thanks again.
The knife I first saw this on was a long bladed hunting knife, probably 7 ot 8 inches, with a hilt and deer antler handle. But the sheathe fascinated me the most. I have since bought a commercial knife made in Finland, called a Puuka, that has this center seam sheath made for it, and a special synthetic material for the handle that does not get slippery when it is coated in blood. The blade is only about 4 inches, and maybe 1/2 inch wide, and it makes a fine knife for gutting and skinning deer. The puuka has no hilt, but does have a brass endcap, and I don't worry about my hand moving on that handle when it gets bloody. I do curl my little finger behind that endcap just as added insurance when I am stabbing or cutting with the knife. This kind of seam, and the added material would make a good choice for a sheath for a knife carried across the back, with a loop provided to go over the belt just below the hilt. The knife and sheath would be under the belt as a way of keeping it positioned and held firmly against the body, so I could feel the knife if it moved in the sheath.