As I understand it...they have changed over the years, but most are 14mm x 1 left hand.
If that's the case, there are several companies making brakes in that pattern, including the YHM Phantom brakes, Spike's Tactical DynaComp, PWS makes a couple, Midwest Indus makes one, etc., etc...there are a lot of them out there. I recently got to play around with a Grizzly brake on a 16.5" VEPR that a friend of mine snagged a while back. It was very nice fit/finish and VERY effective albeit VERY loud (at least on a 16.5" barrel), but absurdly pricey at ~$200. Not sure about their availability, timeline for delivery, etc., but here's a link to the Grizzly ones I'm referring to (my buddy's was a "Ghost Protocol" IIRC): Ghost Protocol Muzzle Brake - For Rifles
