Heater box door explanation?

no idea bout hook up
and no idea about heater boxes in earlier cars as this is based on the unknown bits i have in a box

but you have a door to allow plenum air and wasps to be diverted into the heater or directly out up the leg of the shorts of any passeneger

if you car had no heater at all you would still have the round plenum vent box with this flap and its pull push cable and knob.

you have a flap in the heater box to divert air through the core to heat it up or via a different route so cold air from the plenum can get up to the screen via the platsic manifold and pipes that co up to the dash vents without being warmed in any way.

and you have a flap in there that allows air from the car interior to be pulled in via the box from inside the car and through the core to heat it further

i.e heating/recirculating without the icy wind from the plenum comeing through the heater box

so a pull push for the vent in the front of the round plenum box
a pull push for warmer cooler
and a pull push for outside air or inside air through the heater

one imagines to achieve some of this, a couple of the flaps are paird. so that you can't conspire to make a "dead end"

Dave