Well, look at that column again with a couple brain cells engaged.
For any given year, all models in the letter series tractors 1929 - 1937 shared the first & fourth digits and all models 1938 until ~mid 1953 shared the same first 2 digits.
Model year for tractors with 6 digit serial nrs is determined by subtracting 3 from the first and fourth digit I.E serial nr 300201 [32-3 = 1929].
Model year for tractors with 7 digit serial nrs, 1938 through an unknown date in 1953, subtract 4 from the first two digits I.E. serial nr 4200001 [42-4 = 1938].
During 1953 and to end of production the serial nr assigned to at least some models of letter series tractors was changed to a first digit 6 for Rock Island built and a first digit 8 for Racine built.