4bore, you have an interesting piece there, enjoy!
It the spirit of Det's comments, slug the bore and make an effort to measure the twist while doing so. What I see on the rifling in your last picture makes me think it might have a faster twist, one that would support conical bullets.
#3&4 from the left are belted picket style bullets, .50 caliber. They can work with a flat path or paper cross strips depending on dimensions of bore/bullet/patch, and the nature of your bullet starter.