ok..here's is the deal... The HP umpire called it a HBP, when it was in fact a batted ball, because he ( HP ) called it HBP, we can't assume the batter would have been thrown out. In this play we should leave the ball in play, the the defense make a play, then if you don't have replay you might have to get together and make a ruling ( which is ok)...
this play is a do over, batter come back with the same count.
SLow down your timing and let things play out. some of our schools have replay, i just had this exact play happen 2 weekends ago and we went to replay, turns out it did hit the knob..ball rolled foul, so no harm no foul...LOL.
here is a great tip for HP umpires, when the batter steps in the box, look at his hands and see how he is holding the bat. if they choke up, then its a good chance it hits the knob, if they are wrapping their finger around the knob, well, it's a good chance it hit the hand.