I am glad to see Trix or Treats cards from Set B continue to bring sustainable prices. The seller mischaracterizes the manufacturing time lines. The first set Rosen issued was Set A, comprised of six designs. These are larger, more colorful cards and were issued in the 1930s. Set B, from which this card comes, was released beginning in the later 1940s. As with Set C, issued in the early 1950s, five designs comprise a full set. By and large, although cards from Set C are more difficult to find, cards from Set B typically fetch the most money. A full accounting of these sets can be found on pages 90-92.
Mark B. Ledenbach's vintage Halloween collectibles blog.