quilting momma,
The pitman arm is the shaft that runs from the upper right corner of the treadle foot plate to the crank at the wheel.
Some are made of wood, some are made from metal. The metal ones have to be lubed and adjusted to run right.
Here is my #2 66 treadle:
If you look at the upper right corner of the treadle plate you'll see the ball socket nut of the new silver pitman arm. It goes up to the crank in the center of the wheel.
Joe