Section 121(3)(b) in The Naval Ceremonial, Conditions Of Service And Miscellaneous Regulations, 1964
(b)Acting Sub-Lieutenants. - On completion of [6 months'] [Substitued by S.R.O. 237, dated 24th May, 1975] service as Midshipmen, and subject to passing the Seamanship Examination and being recommended for promotion, Midshipmen shall be granted the rank of Acting Sub-Lieutenant with seniority of the date on which they are promoted.