Not sure about '73, but earlier control switch innards were made of soap stone, same stuff used for marking layout lines on steel. The soap stone pieces were cut in saw tooth fashion so that a pushed button moved the soap stone sideways to the bottom of it's tooth, actuating a vacuum line.

Soap stone pieces are slick (like soap) and low friction, but they eventually wear out after so many button pushings. They can be DIY rebuilt with new soap stone pieces of the correct silhouettes - if you can reverse engineer what the original shape was.