Yes, a Mountaineer stock should fit an Extreme with no problems. Some Extreme stocks will also fit a Mountaineer.. Older ones will not wihout some minor inletting for the recoil lug but Elite and LRH stocks will.
Mountaineers, LRHs and Elites all have the "cantilever" recoil lugs. Some of the newer Knight Extreme stocks also have the extra cutout for this type of recoil lug.
were could I buy a knight disc extreme master hunter (45) straight stock in left hand. pretty much same as right hand only safety
is on left side. rifle has a thumbhole stock now I bought it new right before knight closed down and no black stock was offered for
the lefty right hand received both. thumbhole is lugged also .