315. If the ship, in respect of which a Passenger Ship Safety Certificate is in force, or a Special Trade Passenger Ship Certificate and Special Trade Passenger Ship Space Certificate together with a Passenger Ship Safety Certificate are in force, has on board in the course of a particular voyage a total number of persons which is less than the number for which life saving appliances on the ship provide and less than the number stated in the Special Trade Passenger Ship Space Certificate, the owner or master of the ship may obtain from the authority who issued the certificate a memorandum to be attached to the Special Trade Passenger Ship Certificate or the Passenger Ship Safety Certificate stating the total number of persons carried on the ship on that voyage and the modifications which may be made for the purpose of that voyage in the particulars with respect to life saving appliances stated in the relevant certificate.