Dealer1
See below:
I think the pieces you are missing are those on the red towel. At the junction of the arms is a threaded plate that the vacuum shaft screws into.
The other two photos show the position of the horn relay which is just below the voltage regulator.
The photos of the top pieces are part of my project car and will go with it when I sell the car.
I think these parts may be found on all GM convertibles of the forties except for Cadillac.
They may be hard to find.
Best,
Charlie

BTW: The length of the arms are the same. The photos make it look like they are two different lengths but they are not. One for the left and one for the right.