In computing years of service as a district attorney for any purpose under this chapter, up to three years of service as a member of the General Assembly and any time served as an attorney for the state or any authority thereof on a salary basis may be counted in computing the number of years of service required of a district attorney, provided that he pays into the fund the maximum payment provided for by this chapter for each year of such service as a member of the General Assembly, and any time served as an attorney for the state or any authority thereof on a salary basis, plus interest on such payment at the rate of 6 percent per annum. Such payment shall be made on or before June 30, 1974. In computing such credit, any such district attorney shall be credited for a full year for each year of membership in the General Assembly and a full year for each year he served as an attorney for the state or any authority thereof on a salary basis.