Bob, using your photo, you can make out the fins of the heater core in the space forward of and below the flap. The flap is in the horizontal position, thus closing air access to the distribution box from the top. In this position, the heater core valve should be open.
When I redid my entire system, I kept in mind the "heat in the cabin problem even with the valve closed" problem I was having. Among several other things I did to try to remedy this, I wanted to eliminate air flow across the core even with the valve closed. I did this because I think that the core remains warm even with the valve closed due to heat conduction at the valve. I also noticed that the rubber seals around the flaps are actually doubled-sided and are designed to seal off air flow both when the flaps are shut vertically as well as when shut horizontally. So with this in mind, I made sure to set up my cables and levers so that my flaps will seal tightly when vertical against the metal edges that define the opening for air to flow toward the heater core. The net effect of this is that when my flaps are vertical, the lever knobs are not fully max'ed out against the ends of their slots so that enough sealing pressure can be applied.