A very hard to find piece of hardware
The world of aircraft construction uses drilled-head bolts to secure things that may otherwise vibrate loose or cannot accept a locking nut. I've used drilled head bolts with safety wire to secure a wooden propeller to the threaded hub.

In this application, it would appear that the cable passes through the length of the bolt and the threaded hole in the head would accept a set screw. 

I'm inclined to agree that they items would have been custom made for the manufacturer, but that would not preclude having some made, especially in a small CNC shop.

It seems unlikely that they would have to be particularly strong (as in aircraft grade, or high carbon steel, or grade-8) and could even be done by a hobbyist. I have a "toy" lathe from Harbor Freight that would handle the 3/16" through hole and a "toy" milling machine from another source that would manage the set screw hole easily enough.

You could save a bit of labor by purchasing existing drilled head bolts, although those may be quite strong and difficult to machine.