The first catalog to feature the Model 21 Duck Gun, 5 years after the 3-inch magnum was introduced in the Model 12 and about 10 years after other doubles.  Before 1940 if a customer wanted a waterfowl gun he had to special order a gun with 3-inch chambers, recoil pad and a shorter length of pull.    I find this puzzling when the effort to build a Model 12 for the 3-inch shell is compared to simply lengthening the chambers of the Model 21 and fitting a half inch shorter stock and recoil pad; let alone that the Olin's were avid waterfowl hunters.  I wouldn't be surprised if not a few were built before 1940 for company executives and friends, much like the 20-gauge 3-inch, most likely in the higher grades.