George Andrix, It might surprise a lot of folks but many Native Americans often sharpened only one side of their knives. A single bevel on the left side of the blade for left handers and right side for right handers works quite well for cutting patches. The reverse for scrolling out meat when making jerky. A planer blade makes such a single bevel knife. Straight razors for shaving are tempered much too hard (65+ RC) and will chip with repeated sliding across the muzzle.
Thanks Old Sparky for sharing how you haft the San Mai style blade. It is difficult for many to find the pre drilled hole/s in a rat tail tang from the outside of the antler handle. Measure and mark carefully and hope for the best. In pre-epoxy days pitch would have been used but a cross drilled pin to secure it was good insurance. For a full rat tail tang peened at the butt end over a metal cap the tang can be bent to match the curve of the antler and heated red hot then burned in. Two blind holes drilled from each end of the antler and hopefully meeting close to the middle make the burning easier. A relatively straight antler handle is less worrisome as a straight unheated tang can slide into a straight hole..
While there weren't dedicated patch knives per se, you can find knives and sheaths attached to many old hunting pouches. Check out Madison Grant's the Kentucky Rifle Hunting Pouch. Might find some good examples for inspiring your design. Opinels made in France are handy and cheap but the chrome plated, twist lock mechanisms are too modern for period friction folders. Those who aspire toward authenticity may elect to avoid them and use the more dangerous, non locking friction folders.
Non locking spring back folders like a Barlow that "walk and talk" could be kept in the shooting pouch. I know of a flint shooter in Colorado who uses his big Bowie to start the ball and blade to cut the patch and can do it in half the time it would take for someone to take his folder out of his bag, open the blade, cut the patch, fold the blade back and replace it in the bag. A fixed blade in a easy to reach sheath seems more efficient.
Not everyone is into re enactment so what works for you is what you should carry and use. A patch cut on a thumbed ball off the muzzle is no more accurate than one cut on a loading or bullet block. You can load from the block twice as fast as the muzzle cut patch method.
