A typical 21 skeet gun may have "skeet" stamped on the receiver ahead of the trigger guard. But when you look at the choke stampings just ahead of the flats you'll see WS1 and WS2 on 12s, 16s, and 20s - not skeet. Usually right barrel WS1, left WS2 - though some are reversed. I've got one that's WS1 and the WS2 is factory overstamped/opened to IC.

Chokes on 28s and .410s will be stamped skeet - and not WS1/WS2.

Sam