46 C.F.R. § 131.835

(a) Except as provided by paragraph (b) of this section, ach portable fire extinguisher must be marked with a number, and the site of its stowage must be marked with a corresponding number at least 13 millimeters ( 1/2-inch) high.

(b) If only one type and size of portable fire extinguisher is carried, the number may be omitted.