Being of the machining trade, I'm puzzled as to why would anyone make a plug gauge out of lead. The first use would damage it. By the post Civil War era, machining had gotten pretty sophisticated. The Colt SAA and even some of the muzzle loading predecessors were machined nicely and have some difficult machine cuts in them. The history of measurement as it applies to inspection gauging is very interesting to me. Although I haven't found any particular evidence to support it, I think measurement methods were getting pretty sophisticated by the post Civil War era as well.
