I had that same engraving pattern on the lock of a percussion Gocher trade rifle I picked up in 1958 and the lock looked very much like yours pictured here. The gun was a composite of a Tennessee style cherry wood stock with sand-cast brass furniture. Someone also added brass inlays of thin material in common Indian shapes, including a 'pointing finger'. What you need to do is measure the distance from the center of the hammer gear to the nipple; this will be the "throw". Get Dixie on the phone and beg for help with that dimension. You might also try muzzleloaderbuilderssupply.com, they have been very helpful on a few of my projects. May the force be with you.