12.  The term of eligibility shall be such as the Commission may by regulation determine, but shall not be less than one year from the day on which the name of the eligible is entered upon the register:  Provided, That for public and sufficient reasons the Commission shall have authority to extend the term of eligibility of the eligibles on the register of any State or Territory for such period, not exceeding one year, as it may deem necessary, without correspondingly extending the term of the eligibles on the registers of the other States and Territories as to which the same reasons do not exist.

RAILWAY MAIL RULE IV.

1.  All vacancies in the classified railway mail service above class 1, unless among the places excepted from examination, shall be filled by promotion, upon such tests of fitness as the Postmaster-General, with the approval of the Commission, may prescribe:  Provided, That a vacancy occurring in a State or railway mail division in any grade may be filled by the transfer of a clerk of the same grade from another State or division, under such regulations as the Postmaster-General, with the approval of the Commission, may prescribe, or by reappointment under the provisions of Railway Mail Rule VI.

2.  All vacancies in class 1, unless filled by transfer or reappointment under Railway Mail Rule VI, shall be filled in the following manner: 

(a) The general superintendent shall, in form and manner to be prescribed by the Commission, request the certification to him of eligibles from a State or Territory in which a vacancy then exists.

(b) The Commission shall certify from the register of the State or Territory in which the vacancy exists the names of the three eligibles thereon having the highest averages who have not been three times certified:  Provided, That if upon said register there are the names of eligibles having a claim of preference under section 1754, Revised Statutes, the names of such eligibles shall be certified before the names of other eligibles of higher grade:  Provided further; That if there are not three eligibles upon the register of the State or Territory in which the vacancy exists eligibles may be certified from the register of any adjoining State or Territory.

(c) The name of an eligible shall not be certified more than three times.

3.  Of the three names certified to the general superintendent one shall be selected and designated for appointment, and more than one may be if there be more than one vacancy existing at the time.

4.  Each person designated for appointment shall be notified, and upon reporting to the proper officer shall be appointed for a probational period of six months, at the end of which period, if his conduct and capacity be satisfactory, he shall be absolutely appointed; but if his conduct and capacity be not satisfactory he shall be so notified, and such notice shall be his discharge from the service.