Monark serial #'s were stamped on an aluminum plate, riveted to the BB. The frame can be dated by the s/n. They only used the plate from 1946-54. In 1955, they introduced a new middleweight line, which replaced all their balloon models. These 55-57 serials are different and to my knowledge haven't been decoded. In late 57 Huffy bought Monark, and following models started as a mix of Huffy and Monark parts, becoming just rebadged Huffys with Huffy serial #s by around 1960. -Adam