Not sure if it goes for all V1s, but I know early Mayhems used Spyder threaded barrels while all V2s were Cocker threaded, feednecks were advertised as Angel threads.
Some super low profile feednecks will NOT work with this breech as they must also pass thru the body "sleeve", you'll have to experiment on that if you want to replace it. You could always hack it down and attach a Planet\Macdev style clamping feedneck.
I know very little of the internal changes (if any) between the V1 & V2 Mayhems. They have a monster of a bolt on the inside, unlike the Novas which had none. The solenoid is the same Clippard model used on Nova ETs, the Darkstar, both versions of the Mayhem and I think even on the Assault 80s...