Should a disability beneficiary under age fifty-five be restored to active service at a compensation not less than his average final compensation, his retirement allowance shall cease, he shall again become a member and he shall contribute thereafter at the same rate he paid prior to disability. Any prior service certificate on the basis of which his service was computed at the time of his retirement shall be restored to full force and effect and in addition upon his subsequent retirement he shall be credited with all his service as a member.