The cessation of the employment of a contributor by the Commonwealth, being a contributor who is the holder of a statutory office under the Commonwealth, by reason of the expiration of the term of his appointment to that office shall, for the purposes of this Act:
(a) in the case of a contributor who has not attained the age of sixty years and does not desire to be re - appointed to that office--be deemed to be discharge;
(b) in the case of any other contributor who has not attained the age of sixty years--be deemed to be retrenchment; and
(c) in the case of a contributor who has attained the age of sixty years--be deemed to be retirement and, if the contributor has not attained the age of sixty - five years, be deemed to be the retirement of a contributor the age for whose retirement is fixed by law at an earlier age than sixty - five years.