Section 10. If an employee serving in a position which is classified under chapter thirty-one or in which he has tenure by reason of section nine A of chapter thirty shall be appointed to a position within the department of any agency under the department which is not subject to the provisions of chapter thirty-one, he shall upon termination of his service in such unclassified position be restored to the position which he held immediately prior to such appointment; provided, however, that if his service in such unclassified position shall have been terminated for cause, his right to be so restored shall be determined by the civil service commission in accordance with the standards applied by said commission in administering chapter thirty-one. Such restoration shall be made without impairment of his civil service status or tenure under section nine A of chapter thirty and without loss of seniority, retirement, or other rights to which uninterrupted service in such prior position would have entitled him. During the period of such appointment, each person so appointed from a position in the classified civil service shall be eligible to take any competitive promotional examination to which he would have otherwise been eligible.