5 U.S.C. 3321 provides for ``a period of probation . . . before initial appointment as a supervisor or manager becomes final.'' It also says that a supervisor or manager ``who does not satisfactorily complete the probationary period . . . shall be returned to a position of no lower grade and pay than the position from which the individual was transferred, assigned or promoted.'' This subpart contains OPM regulations implementing those requirements of law.