Hi Feltwad,
I tried to post this earlier but the ALR site was down. It is a nice gun and thank you for posting it. I've seen examples of the the top breech plug English rifles that go back to the 17th century. If you add bottom loading breech loaders, I've seen more early 18th century English breech loading rifles than muzzle loading examples. On your example, the trigger guard, checkering, and lock are from the very late 1760s and more likely the 1770s. For example, the guard has an acorn finial, the coarse checkering is right out of the 1770s or later, the lock has a short sear spring. Surely, you know those features are not from the 1750s. The feather spring must be a replacement because I cannot believe a quality British gun maker would make one like that. It is an interesting rifle but it has no clean 1750s provenance.
dave