OK, I finally got a chance to take some pics last night. Thanks to B for providing the chance.
Here's a pic of the J-tube. This pokes up through (barely) the engine tin, and carries air from the heater blower to the heat exchanger. On stock 73-74 cars, the right-side one is capped off and only the left-side one is connected to the blower.
Here's a pic of the S-duct. It connects to the outlet at the bottom edge of the fan shroud, and carries air from there into the heat exchanger proper. The J-tube joins the S-duct in the middle.
Here's a pic of the two parts joined together.
Here is a pic of the flapper valve and the tube which the valve is attached. This is the outlet for warm air from the heat exchanger. Air gets pushed through the tube to the flapper. When the flapper is closed, it dumps air out underneath the car through that mushroom-shaped thing toward the bottom of the picture. When the flapper is open, the air instead is directed to the other opening, toward the top of the picture. That opening has a hose clamped to it that runs to the heater duct in the car's body.
Here are the bits joined together.
Here is a closeup of the area where the parts are joined. They are not connected in this pic, just laid out in the right places.
...I also have the pics in 1600x1200 size if you
really need them...
--DD