46 CFR § 131.835
Portable fire extinguishers
April 15, 2021
(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.