Leopold & Loeb
Once inside the car Bobby was grabbed from behind and bludgeoned with a taped steel chisel.
This is probably what happened to the rest of the black tape. It was used to wrap the actual murder weapon, which wasn't the flashlight, ball bat, golf club or hearth tool. Those would not have made the fracture with void as shown on the diagram. Possibly it was some sort of specialty tool, perhaps used in the killers normal business or hobby but wrapped to minimize external signs of the wound. Perhaps it was bought at a hardware store and modified or was a combination of two items put together. A 1/2" pipe, about 30" long, filled with lead and with a "Tee" fitting at one end, flattened with a sledge hammer, and taped. A custom made "weapon" like this could be dismantled and/or discarded fairly easily I would think. A plumbing contractor would have all this. Or it could be bought by anyone at a hardware or home improvement place.
A sketch of what I think the weapon looked like
Drawings of the fractures as I think they would look if caused by the flashlight
Knives Made from railspikes by craftsman in Silverado