Did you Know? (July 2023) Posted on August 7, 2023August 7, 2023 by Guy Gasser The Scott Catalog for US stamps does not use numbers 520-522. This is because those numbers first listed the first three airmail stamps, which are today C1-C3 before airmail stamps received the “C” as part of their number and their own section in the catalog. The editors never went back and reused these numbers after the move. Oops? Grow Philatelty - SHARE!!