Section 131.835 - Portable fire extinguishers(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.