Very interesting gun. And, the reference to Indiana jogged my memory. I have a gun that the family swears was built in Lakeport California, the only problem is the barrel is stamped J.Rogers Elkhart Indiana. But the Elkhart Indiana stamp is over stamped with the J, Rogers stamp. So, I suspect John Rogers while on his trip to San Louis Abispo California, where he eventually established a gun shop, stopped in Lakeport, and possibly stocked up a rifle from used parts to sell, and possibly getting those parts he needed from Charlie Slotterbeck.
The gun I have is a walnut half stock with no cheek piece, a little perch belly on the underside of the butt, a relatively short forearm, with a small poured nosecap, and a single barrel key positioned on the forward end of the forstock. My gun has no underrib which creates a problem when you need an under lug, or a staple, for the barrel key. Charles Slotterbecks guns traditionally have no underrib. The workaround on my gun is that the barrel is heavy 1 1/8” across the flats, and only .36 caliber, so a slot for the wedge was cut through the bottom flat of the barrel. The triggerguard, an buttplate are iron, and the same style as those used by Slotterbeck, but a little more robust than those used in Lakeport. I suspect these parts may have been left over from when Charlie worked in San Francisco, when his guns were a bit more robust.
So what I’m saying is if I were going to start looking for a possible builder of your gun, I would start in Elkhart Indiana.
Hungry Horse