I have a rifle with a similar signature and here is what one of our members had to say about it- “The barrel signature sure looks like one of Sam Loudenslager's. He was a prolific gun maker. Most of his guns came out of his shop/farm near Mexico, PA in Juniata County. He learned the trade in Union/Snyder County near New Berlin / Selinsgrove area. He made single barrel & swivel-breech longrifles. FYI... the wriggle engraving on the bow and extensions of the trigger-guard (as seen in the ALR Library photos of his other guns) are almost a signature feature. I would definitely like to see more photos of your old gun. Does your rifle retain the "Roman-nose" buttstock profile typical of guns from Central PA? I would also be curious to know where your GGG Grandfather lived. Sam Loudenslager's guns turn up all over the country. One of his rifles is one display at The Henry Ford Museum in Michigan.” Thanks for posting the photos.
Looks like you've probably found your maker!