Winchester was making and heavily advertising Model 21 Skeet Guns by 1932. Here is the page from the 1933 pocket catalogue --



In the early years the Skeet Gun was offered in Tournament, Trap and Custom Built Grades. For 1936, the term Tournament disappeared and it became Standard Grade. From the 1938 Winchester pocket catalogue --





By 1940, the Trap Grade was gone and it was just the Model 21 Skeet Gun Standard Grade. By 1946 it was the Model 21 Skeet Gun. There never was a Model 21 Skeet Grade.