Notwithstanding any other provisions of law, effective July 1, 1964, the rates of Class Two employer contributions shall be revised on the basis of actuarial valuations to reflect the additional cost resulting from the amendments effective on such date. Such revision shall be made over a three-year period commencing July 1, 1964 so that at the end of such period the ultimate contribution rate of Class Two employers (currently computed to be seven and one-half per cent in the case of the State and six and one-half per cent in the case of other Class Two employers) shall be achieved in accordance with actuarial valuation. The Board shall establish such rules as it may deem necessary and equitable to all Class Two employers for this purpose.