Ever seen a bolt like this?

[mancad]

This came out of a 1968 C20 4spd transmission side cover. The threads would not come clean with a wire brush so I got a small screw driver and picked it clean. The residue in this area was brown/black, fairly tough, and fibrous. When cleaned, it showed this notch machined into the threads. All 12 bolts were the same. I'm guessing they pressed in a piece of plastic or fiber to keep the bolt from loosening; maybe it helped seal the hole as well. I've not run across this before. It must have been a fairly expensive solution.