Help with a mark on an 1806 25c B-10 please.
I noticed that both of my B 10's have a similar mark between the 2 and 5 on the reverse. Looking in Tompkins book there's the same mark however in the Rea Peterson etc
The mark is not visible. Could this be a clashmark ,a die chip, a result from the die bulging, an engravers mark or something totally different?