There was a tool for holding older motorcycle cylinder heads in a vice at one time, it was just a hex bar with a different sized spark plug thread at each end.
Saw the same bloke at another autojumble today. He had another autovice.
I asked what the holes were for and he said he had been told the were for cropping rod. Trouble is they are blind holes meaning the amount of rod you coud crop woud be limited.
However i noticed the vice he was selling had another part not on mine. A removable jaw that fits into the pipe vice presumably for smaller diameter pipe. Basically a bit of flat bar with a v shape cut into it (shoud be easy enough to fabricate) but i noticed this had a hole in it.
I wonder if the holes on the side were just to hold the ancilliaries when not in use?
Doubtful. Would be about as useful as a chocolate fire guard. First bit of movement and whatever is in there would likely slip/drop out. Are you sure they've never been threaded? Some ape might have smoothed the threads through cross threading or suchlike.