The board of examiners at the New York customs district may at any time,
with the approval of the Civil Service Commission, order an examination
for promotion, and at least five days before the examination is to take
place shall cause a notice to be posted conspicuously in the office for
which such examination is to be held, and shall state in said notice the
class or classes to test fitness for promotion to which the examination
is to be held and the time and place of examination. Promotions shall
be from class to class, and the examination of persons in one class
shall be to test their fitness for promotion to the next higher class:
_Provided, however_, That if in any examination for promotion the
competitors in the next lower class shall not exceed three in number,
the board may, at its discretion, open the competition to one or more
of the classes below the class in which there are not more than three
competitors. All persons in the class immediately below the class for
which promotions are to be made, and who have been in said class at
least six months, must be examined for promotion.