Looks like, as you say, a well executed 1960's era custom job. It's a far cry from a "Bubba". Don't forget that in this period every sort of Mauser , Springfield, and Enfiekd action was being used to build custom rifles. Quick question: cock on opening or closing? Best guess... Local custom smith who figured out "how" to do these semi cookie-cutter and who built a few. "Probably" not done in bulk for a reseller. Often times a local Smith will make one, then a buddy wants one too, and before long he's done ten. A local guy here did just that about 20 years ago with full length stocked Krags, of all things. Ended up building at least twenty of them. Local specialty. Without a stamped make it'll be hard to track down. The Remington 660 style bolt is distinctive. Interesting rifle. Bet it shoots well.