The 3¢ Admiral has a set of mystery markings which are most easily recognized by the two parallel lines which slope down and to the right from the central portion of the left '3'. These marks (and several others) were discussed in Mystery Markings on Admiral Stamps, by Leopold Beaudet and Richard M. Morris, which was published in The Admiral's Log in January 2008 (Vol.XI, No. 1).

In the article five characteristics of these flaws were outlined:

1. A horizontal line running through the top part of the bottom left leaf
2. Lines in the leaf stem closest to the LNB
3. A pair of short diagonal lines to the right of the upper part of the "3" in the LNB
4. A second pair of diagonal lines on the right side of the "3" in the LNB. This pair is much longer and occurs much more frequently than the first pair.
5. Two horizontal dashes in the bottom margin below the "3" in the LNB

These flaws repeat on many subjects on several plates. Their appearance is similar from subject to subject but their position varies. Flaws 4 and 5 are the most common. All the flaws are apparently related because two or more of them are often found on the same subject.

These flaws are also found on both the 3¢ Brown and the 3¢ Carmine.