The same authors contradict themselves in the red book, giving National dates (in more instances than the BB) of the earliest 1914 to 1930 for most and 1940s for some. In that book they also date the dual WWSO logo with the c/n as 1930s to 1940s, but the c/n is generally presumed to have come into use on Wagner pans around 1924. The problem lies in that older catalogs show illustrations not photographs, so pinpointing years is near impossible. There's no reason to believe a 10 year gap existed in the production, so I would discount the BB dating as proofreading error. Since, except in only the rarest of instances, Nationals are of the outside heat ring design, I tend to think that if they made it to the 1940s, it wasn't for very long. Catalogs I have dated 1924, 1927 show them, and an aluminum catalog dated 1936 has National branded pieces. A Wagner catalog presumed to be from 1941 does not show them at all.