When the Model 21 Skeet Gun was introduced in the 1933 Winchester Catalogs, it was offered in Tournament, Trap and Custom Built Grades. Those remained the offerings through 1935.
Skeet Gun 1935 straight grip.jpeg
Skeet Gun 1935 pistol grip.jpeg
Note the pistol grip Model 21 Skeet Guns have "field" dimension stocks while the straight grips have higher combs.
Between 1935 and 1936, Winchester eliminated the Tournament Grade Model 21s --
Skeet Gun 1936 straight grip.jpg
Skeet Gun 1936 pistol grip.jpg
The extra cost option of a ventilated rib on Model 21 Skeet Guns is first mentioned in the 1938 Winchester catalog. The Trap Grade was dropped between the 1939 and 1940 Winchester catalogs.