I've been rebuilding the entire heater system on the car for a few months now, at a very leisurely pace (house move also got in the way). I blanked off all the apertures in the bulkhead so I could still use the car, albeit without aircon or heating.
I removed the heater box and heater control box to find most of the foam lining had disintegrated and all of the rubber bushes had perished - not surprising after 45 years.
I'm currently doing a trial run refitting the heater control box, which, as you will know, contains the lower temperature flap. I want to make sure it all works in situ before fitting the control box permanently, i.e. using sealant around the rubber gasket to make sure no engine gases enter the passenger compartment. I don't fancy digging it out again.
I intend to spend tomorrow morning fault finding so I will include your suggestion.
I will report back.