Just got mine out of jail and I have the same problem,shredded o ring.Not from the group buy serial number MU97xx.
KAC was apparently able to recreate the issue with my piston and it looks like they just opted to replace it. Just got the new one and it threads on properly. Now I just have to wait for the form 4 to get approved.
Did you send WHOLE can back? or just piston. And why are you waiting on a 4 if it's just a piston? Forgive my ignorance.