According to the Ford Parts Price List, the special headless lock screw that secures the distributor is an A-20928-S8 which is 3/8-24 x 11/16 and zinc plated. The lock nut is A-21750-S8 hex with a 3/8-24 thread. This info appears in the Price Lists of January, 1928 and July, 1931 so the screw & nut are the same for 1928 through 1931.
