Afaik, and I could be wrong.... (I've only ever had 2 of those in my hands) the thread pitch on those is the same as a Nork castle nut. This is of course conjecture as I didn't have the presents of mind to actually measure one when the opportunity was had!
I would imagine the only way you will find a jam nut is to make one, or have one made.
Personally, I always thought those factory DA socom brakes were pretty cool looking! Useless as tits on a boar, but pleasing to the eye! Curious what happened to the factory brake, and where the previous owner sourced an aftermarket one with the correct threads..... Custom job perhaps?
You could turn the barrel down to 1/2"x28, but that's a waste of a somewhat limited availability of a DA socom..... There don't seem to be many of them around.... Whereas a 22" cut to 18.5 is fairly common in comparison.
Alternatively, you could trim a bit off the brake so that it indexes properly with-out a jam nut..... I wouldn't recommend doing it without a lathe.
Any pics of the aftermarket brake?
