The lockplate normally doesn't get changed out when converting to percussion. The pan is cut off the barrel. (You can see the remainder of the brass pan in the photos). A drum or bolster and nipple are added, the hammer is replaced and the frizzen mechanism is removed. The left over holes in the plate are just filled. As for the name being "covered" by the frizzen and to the different size of the marking, I would propose that as these were hand made, anything would be possible.