In computing years of service as a district attorney for any purpose under this chapter, any time served by a district attorney as a judge of any county court from which appeals can be taken directly to the Court of Appeals of Georgia may be counted in computing the number of years of service required of any district attorney, provided that he pays into the fund the maximum payment provided for by this chapter for each year of service actually served as such a judge, plus interest on such payment at the rate of 6 percent per annum. Such payment shall be made by not later than January 1, 1975.