And I guess another complicating factor is that the catalog doesn't show how long a given stamp remained available. For example, during the years when the first-class rate was 6c, there were no 12c stamps issued to pay the two-ounce rate--but I suppose #1286A from a couple years earlier might still have been around? Or were you just expected to use two 6c stamps?
Additional ounces cost the same through the inception of the 10 cent letter rate in 1974. During the 10 cent rate period, the additional ounce rate was reduced to 9 cents in September 1975, creating a differential which has remained.