From and after January 1, 1962, the classification of counties shall be fixed and governed by the assessed valuation as finally fixed for the preceding year. Provided, one hundred twenty days after January 1, 1962 and one hundred twenty days from January 1 of each second year thereafter, the classification shall be determined by the secretary of finance and administration from the assessed valuation of each county as finally fixed for the preceding year, and the secretary of finance and administration upon making the determination shall notify the board of county commissioners of each county of the class within which each of the counties of this state falls according to the classification, and the classification as so fixed and determined by the secretary of finance and administration shall govern the salaries of the county officers for two years beginning July 1 of the year the classification is determined.