A closer reading of the 1933 catalog shows "The Model 21 Winchester Skeet gun is furnished in the Grades mentioned above."
Those grade are Tournament, Trap and Custom Built. With the advent of the 'SKEET' gun in 1936, it appears that another Grade was offered marked simply 'SKEET' on the floorplate and so marketed. Guns before 1938 appear to have a comination of Tournament-SKEET, Trap-SKEET. I have never seen a Custom Built-Skeet.
It was over 70 years ago.-Dick