For the privilege of selling tangible personal property at retail a tax is hereby imposed upon all retailers at the rate of 21/2percent of the gross receipts of any retailer from the sale of all tangible personal property sold at retail in this state on or after August 1, 1933, and to and including June 30, 1935, and at the rate of 3 percent thereafter, and at the rate of 21/2percent on and after July 1, 1943, and to and

including June 30, 1949, and at the rate of 3 percent on and after July 1, 1949, and to and including July 31, 1967, and at the rate of 4 percent on and after August 1, 1967, and to and including June 30, 1972, and at the rate of 33/4percent on and after July 1, 1972, and to and including June 30, 1973, and at the rate of 43/4percent on and after July 1, 1973, and to and including September 30, 1973, and at the rate of 33/4percent on and after October 1, 1973, and to and including March 31, 1974, and at the rate of 43/4percent thereafter.