“ Any enlisted man of the Navy detailed to perform the duties of ‘‘ship's tailor’’ on board of a vessel having a complement of six hundred men or more, exclusive of marines, shall receive twenty dollars per month in addition to the monthly pay of his rating; on a vessel having a complement of from three hundred to six hundred men, exclusive of marines, fifteen dollars per month in addition to the monthly pay of his rating; on a vessel having a complement of less than three hundred men, exclusive of marines, ten dollars per month in addition to the monthly pay of his rating. ”