Here are two “mini” papers I wrote about shield cent doubling. How to spot it, where to look. Also issues that one can come across. Now what I see a lot people will say doubling is present in a shield cent. (That person is looking for the old style doubling like the 1955 or 1972 style doubling.) that’s not what we want we are mainly looking for extra thickness and that is it. Spilt serifs (notching) will accompany that thickness but at that point the doubling will be noticeable extremely extra thick.
