Bob (I think) Day was an old longrifle maker in Chilliwack BC. My .45 longrifle is one of his. Beautifully inlet and with excellent wood, it displays several schools of building, and has a straight octagonal 7/8" bl. It was .50 cal when I aquired it, however the hole was not centred and each sight had to be way over at the end of reason to sight in. I re-barreled it to .45 and it was the orignal test bed for my .40 barrel was well.
The gist of this post, is that the treaded plug was a short schedule 5 5/8x18 socket plug screw. The tang has a projection that fit into the hole of the socket screw plug. What held the barrel in longitudinally and back against the tang, was the barrel pins. This system is safe and has worked for many decades. "Day' built rifles over along period of time and as far as I know, no failures.
This rifle was the first flint rifle I 'kept' and I got it by trading an Italian Sharps actioned, Christian Sharps barreled Carbine .50/70 to CB, a long time ago. It was my first 'kept' flinter and taught me to shoot flint - I'm still learning and it still is a favourite to shoot. It still makes nice 1/2" groups at 50 yards off the bags.