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Mine is from 1947 and measures .008" before the flare. The point of maximum constriction is a good bit closer to the muzzle than 3" (Crossman) or 2 1/4" (Ithaca drawing). They do vary considerably.

Left is WS-2 and measures .015". I have no idea nor have I heard a satisfactory explanation as to why Winchester considered .015" applicable to the game of Skeet.

The gun is choked perfectly for doubles from 4. If all you shot was doubles from 4, you would have as they advertised "Your Gun for Skeet".

Since reaming on my original Model 21 seems unwise no matter how silly this "Skeet gun" is choked, I use a spreader in the left barrel for doubles everywhere except 4.

Beretta also uses the constriction/flare profile for their interchangeable chokes marked 'Skeet'.

My two Ithaca Skeet Guns have chokes that mic .009" right and .016" left (NID No. 4E 12-gauge) and .008" right and .014" left (Lefever A-Grade 20-gauge). Certainly way too tight for NSSA skeet. I think a Model 21 with WS-1 in both barrels would be the ticket for NSSA Skeet as shot today. The way skeet was shot back at the time these guns were made skeet was shot going around the eight stations shooting your singles, loading 1 shell at a time. From a 1933 DuPont Skeet Handbook --

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Then you went back to stations 1, 2. 6 & 7 and shot your doubles.

[Linked Image from i.imgur.com]

Back in my younger years while shooting NSSA competition I posted 100 straights in both 12- and 20-gauge with straight cylinder bore barrels.