I purchased a couple of Bell System branded Plantronics Starset headphones off eBay a few months ago. They are both date stamped June 1977. While the sets work, the rubber bushing, which allows the voice tube to be adjusted and kept in place, has deteriorated beyond usability so the voice tube would not stay where the user placed it.
This article shows how I fabricated a replacement out of a mechanical pencil eraser. I chose a mechanical pencil because the eraser was somewhat softer than those usually found in the standard #2 pencil.
I shaved off the sides of the eraser with an Exacto knife to approximately the diameter and length of the original bushing. I'm not very good at fine carving, but my efforts were good enough to work.
Next, while holding the eraser with a pair of needle-nose pilers, I VERY SLOWLY drilled a hole through the center. I chose a 1/32 inch drill bit which seemed to be slightly smaller than the outside diameter of the short tube that sticks out of the end of the Starset. This tube will receive the eraser. I destroyed the first eraser by drilling too fast, so for the second try, I took a full minute to drill through, that worked without destroying the project.
Next I used a Dremel to ream out a roughly spherical shaped depression in one end of the eraser. A spherical Dremel tool would have been a better choice but I didn't have one small enough so I used the next best tool I had available.
I beveled the top end of the eraser on the outside for a better fit inside the cap but probably should have cut a bit more off. Since I didn't have the right Dremel tool, I had damaged the top edge of the cup, so I was afraid to bevel the end any further for fear of destroying the eraser again.
You can see where I slipped with the Dremel damaging the cup somewhat.
While not perfect, it does hold the voice tube in place and allows me to carry on hands free telephone conversations which, when coupled with my Western Electric 2514BM set, sounds much better than most of the headsets available today.