The Serif font is a style of font sometimes used for text in the cancel dial. Serif characters are identified by having a small triangle (flag) at the ends of some letters.

Examples:

Closeup of a cancel from the destroyer minelayer U.S.S. Montgomery DM-17 showing lettering in the Serif font (look at the top of the "S" letters in the "U.S.S." and the bottom of the legs of the "M" letters in "Montgomery")

Closeup of a cancel from the destroyer minelayer U.S.S. Montgomery DM-17 showing lettering in the normal font