It seems while typing a response to the thread on this rifle, iit became restricted for some reason. Here is my response regarding the patchbox engraving spilling onto the butplate:
Here is a possibility.... The box was initially cut out to a pattern which would fit to a flatter butplate. When fitting to the butplate with more curve, the upper sideplate would require substantial shortening. In order to carry the engraving pattern, it was necessary to cheat over into the butplate edge. Perhaps the box and sideplate were reused from an earlier gun?