Putting pieces of tape in a mold is called "beagleing", named after a poster over on cast boolits forum. It's normally done to increase boolit diameter. It also makes the boolits oval shaped.
I'll NEVER do that, I want my boolits round, thank you! If I need the boolit larger, I'll lement the mold, spinning a boolit with abrasive on it in the cavities. That'll remove some of the metal from the mold, if done carefully, the mold remains round.
As for the vent lines, you'd have to talk to a machinist to see if he could run that mold in a mill with IIRC a fly cutter across the faces of the mold.
The more people I meet, the more I love my dog
They're going to get their butts kicked over there this election. How come people can't spell and use words correctly?