I received my 9x21 kit from Numrich today and tried it in several different Witnesses. It's definately not made for the older small frame, 9x19/.40 S&W guns. The slide is held back out of lockup, just like when a large frame barrel it attempted.
It does fit in two of my .38 Super slides and when I mated it to one of my large frame Witnesses it locked up just fine. It does have one issue that I'll have to work out when I get some time to mess with it, which could be awhile. It locks back the slide, via the slide stop, even without a magazine installed. It does this both with the recoil spring installed and without the recoil spring installed, so it has to do with the barrel itself.
This solves the mystery of which frame these barrels fit. It's the large frame.
Once I get the slide lock issue solved, this barrel will become a 9x23 Winchester barrel.
Hope this helps.