When candidates are required for employment in any post in a unit preference shall be given to persons discharged or reverted from such posts in the uni} under Rule 420 so long as such persons desirous of such appointments as available, appointments being made in the inverse of the order in which they were discharged or reverted from such unit :Provided that in the case of person who did not on the date of his discharge or reversion hold the post substantively he possesses the qualifications, if any, prescribed for the post or has, before discharge or reversion been exempted by competent authority from the possession of such qualifications :Provided further that also if the appointing authority considers it undesirable that any specified persons should be appointed under this rule such authority may, for reasons to be recorded in writing and communicated to the person concerned, refuse to appoint him and an appeal lie from such refusal as if it were an order of dismissal.
