Any employe who is a member of the pension fund and is in the employ of the city on the first day of July, one thousand nine hundred sixty-nine, and was a member of said fund for the last five years and any member of the pension fund who has not attained the required age of retirement, but has continued monthly payments as provided in subsection (a) hereof shall have the option of contributing to the pension fund sufficient additional moneys to become eligible for the benefits included in this amendment.
53 P.S. § 23564