Plate Defect Or Extraneous Ink?

Here's a RS41b with a dark line below the R of BROWN'S and a related mark above it in the margin. The ink color appears more like the brown stamp ink than a black ink that might have dripped from a pen. There is no damage or bleed-through or other evidence on the reverse side that is related to these marks.
I have it on ebay and someone has asked me what the mark is. I don't really know, and would like to pose the question here.

Extra ink.

Thanks!