I do belive that the 1976 (and 1974) Traded cards in Topps were simply inserted in packs later on the in season, and were not a separate stand-alone issue, such as they were in the 1980s to the present. Therefore, the 1976 Topps Traded cards are listed as an insert to the 1976 Topps set (so it still falls under Major Release, it just wasn't a major release on its own).
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).