Correction on "unique" acknowledged Shotgunjones -- thank you, I'd forgotten that.

The crux of my post was meant to be that opening a tighter M-21 choke to WS1 isn't the same as opening to standard SK dimensions, and requires something more than a gunsmith merely competent to do the latter.

As for whether Winchester invented it ... well, someone was the first. It's unimportant, but I'd be curious to know of earlier examples.

Jay