(a) OPM shall establish and administer a career-conditional appointment system for positions subject to competitive examinations which will permit adjustment of the career service to necessary fluctuations in Federal employment, and provide an equitable and orderly system for stabilizing the Federal work force. A competitive status shall be acquired by a career-conditional appointee upon satisfactory completion of a probationary period, but the appointee shall have career-conditional tenure for a period of service to be prescribed by regulation of OPM. When an employee has completed the required period of service his appointment shall be converted to a career appointment without time limitation: Provided, That his career-conditional appointment shall not be converted to a career appointment if the limitation on the number of permanent employees in the Federal civil service established under paragraph (b) of this section would be exceeded thereby. Persons selected from competitive civil service registers for other than temporary appointment shall be given career-conditional appointments: Provided, That career appointments shall be given to the following classes of eligibles:

1. (1) Persons whose appointments are required by statute to be made on a permanent basis;
2. (2) Employees serving under career appointments at the time of selection from such registers;
3. (3) Former employees who have eligibility for career appointments upon reinstatement; and
4. (4) To the extent permitted by law, persons appointed to positions in the field service of the U.S. Postal Service for which salary rates are fixed by the act of July 6, 1945, 59 Stat. 435, as heretofore or hereafter amended and supplemented.