Here is an interesting unknown artifact from the lineman's collection: Heavy duty, all metal construction, with metal spacers between the metal discs.

What is it and what was it used for?

Answer: Everyone who responded called it a different name: plug, mandrel, prover or pig, but ALL agreed on how it was used. A winch line was attached to an end and it was pulled through a conduit to clear it of dirt, rocks and other obstructions before expensive cable was pulled through. The device was also useful in determining if the conduit had collapsed.