I read (somewhere on the internet) that the Heavy Duck was not manufactured until 1935. I just purchased one which serial number (806286) implies that it was manufactured in 1931, if the SS/SX M12 serial numbers were contained within the Standard M12 serial number range. It has the forend wood of a 1920's/30's M12 and buttstock configuration of early 1930's era M12s. It's stamped "Super Speed & Super X 3 IN." It has a 32" Full plain barrel with very thick walls and a buttplate rather than a recoil pad. Can anyone confirm the earliest manufacture of the Heavy Duck? Are there different serial number ranges for standards and 3" receiver guns?