(a) Service must be proved by a letter on company letterhead from the owner, operator, master, or chief engineer of a tankship or self-propelled tank vessel. The letter must specify—

(1) The classification of cargo (DL, LG, or both) carried while the applicant accumulated the service; and

(2) The number of days of officer, rating, and cadet service in the engine department on tankships or self-propelled tank vessels; or

(b) Service must be proved by certificates of discharge from tankships or self-propelled tank vessels with the appropriate classification of cargo (DL, LG, or both).