33 C.F.R. § 174.23 – Form of number.

(a) Each number must consist of two capital letters denoting the State of the issuing authority, as specified in appendix A of part 173 of this chapter, followed by—
1. (1) Not more than four numerals followed by not more than two capital letters (example: NH 1234 BD); or
2. (2) Not more than three numerals followed by not more than three capital letters (example: WN 567 EFG).
(b) A number suffix must not include the letters “I”, “O”, or “Q,” which may be mistaken for numerals.