The best way to describe the mechanical timer within the transmission is that it consists of a collection of cams on disks. As the machine goes through its cycle, the transmission changes operation--for spinning, the center post lifts up, allowing the tub to be lifted off the brake, for example. One directional motor; one easily replaceable belt; many individual parts. I've always been amazed that some engineers were able to design this contraption and be able to make it profitable. I still have a collection of these transmissions in my cellar.
That 1948 ad looks like the Blackstone model 50A, but the image looks horizontally challenged somewhat. I sold one of these machines to Greg a few years back. Very heavy machine--it was backbreaking just getting it up 5 cellar stairs. The 250/350s were somewhat lighter, but I still can't imagine how we used to deliver these things to 4th-floor apartments. Taking out the 70-lb. spin tub helped, but still.....
Blackstone coin-ops would date to the 1970s, especially since it was avocado. All the innards were the same, just the coin-operated timer, and an extra ring around the tub to keep clothes inside the tub. I don't actually remember any Blackstone laundromats around here, though.