Heater box dilemma

I decided to convert my 73 Dart Swinger from AC to heat only so have been searching the forums and have discovered a lot of helpful info but now I’m stuck. I picked up a heater box reportedly from a 69 Dart and realized that it has three control doors. Temperature, shut-off, and defrost. That all works great when matched up with the control for that year. I separately picked up a control for a non-AC 73 Dart that has provisions for only two cables. Did one of the doors get eliminated in later year models? I’ve looked at photos of boxes for sale and noticed that some don’t have the shut off door (the one by the heater core inlet and outlet). Can I get away with tying that one back? That just doesn’t seem right but I’m missing something here and would appreciate some input.