Just a guess here, without a Marlin catalog: It was likely in development, pre-WWII. Then along comes the war, production shifts away from civilian guns. Immediate postwar years weren't all that great for firearms production either. (Both Ithaca and Marlin dropped their sxs between the end of the war and 1950.) Things pick up in the 50's, Marlin finally brings it out.
Another possibility: They used the same ad agency Ruger used for the Gold Label, which must've been advertised for a good 4 years before any ever came off the line.
