I'd second Mike - I don't think in any way that's a 1955 rifle. I think someone recycled an older Bill Large marked barrel that had been on another rifle made in 1955. I feel pretty positive that's an L&R Manton lock, and if it had been an original left lock off a double, it almost surely would have a countersink for a cross bolt head on the left side rather than just an exposed thread end; most of the doubles I've seen - the primary historical use for a left hand lock - ran the bolster lock bolt through from the left side and so on the left side there will be a countersink for the head, not simply threads exposed. An original flint lock like this in left hand, in that condition, with no bolt head countersink, would probably be worth more than the entire rifle.
Also the proper wood removal around the breech area (slabbed off straight between breech and lock panels) does not look 1955; almost all those pre-Kindig and Shumway guns were overly chunky with far too much wood left over in that area in particular, rounded and humped.
Please don't take this as overly critical because I think it's probably a 1970s or 1980s rifle, and it's pretty darn nice with pleasing architecture.