I bought one volume of a two-volume set called
Modern Electricity copyrighted 1912 at an antique store a few years back. In it is a block diagram of a telegraph multiplexing system patented in Ireland around the turn of the century that allowed nineteen telegraph circuits on a single (earth-return) wire. The book said that the system worked best during very damp weather since the dampness discharged the capacitance of long copper lines more rapidly. It used a tuning fork (struck every twentieth pulse by an electromagnetic armature to keep it pulsating) to time a stepper relay through 19 contacts that connected 19 telegraph keys to a single wire. An identical device at the other end of the wire sync'd its tuning fork to the originating fork, demuxing the 19 channels to sounders at its location. Pretty amazing: no electronics!
I once knew a very old man who had worked on WU teletype multiplexors. His stories were fascinating, epics of mechanical technical challenges and nightmares. As an electronic tech, I loved his stories. He had also worked as a wireless operator in the merchant marine in the 1930s using an arc tranmsitter. Neat stuff.