The 80's are the first generation, 1961-1965 approximately. '65 was a transition year. The 800's (late '65 - mid '71) are the next step in Scout evolution, with suffixes a and b coming along towards the later stages of production.
My thinking is that IH line at the time began with 100 for the 1/2 ton pickups and went up thru 220 for big trucks....80 was the logical designation for the 1/4 ton Scout.