Should any person holding position in any city of the first class, or any county or other public employe, if any, paid out of the treasury of any such city, as aforesaid, be employed at temporary work, with the understanding that it probably will be for a period of not more than three months, such employe shall not be compelled to pay or contribute toward the pension fund herein provided for; but shall have the option, or choice, of so doing, and, in that event only, becoming entitled to the pension provided by this act.
53 P.S. § 13439