At the first valuation after the effective date of these amendments (in 1959) the actuary engaged by the board of trustees shall compute the rate percent of the total earnable compensation of all members which is equivalent to three and one-half percent of the amount of the total unfunded benefit liability on account of all members and beneficiaries which is not dischargeable by the aforesaid normal contribution made on account of such members during the remainder of their active service. The rate percent originally so determined shall be known as “the accrued liability contribution rate”.