Dear Paul,
Now I am surprised that nobody else has answered yet.
Of course James is right, early cars like yours did not have the opening for the heater valve in the firewall and the ridge was straight all accross. The change to the opening was made somewhere after serial 003100 or so. Certainly your car got new fenders in the past and together with those the sheet metal panel in front of your windscreen was replaced as well. The replacement part always takes care of the valve opening in the firewall with the little bulge in the metal lip you recognized, that's why you find it there somehow "changed" at your car.
It's up to you what you will do with the opening during restoration; my car did not have it and won't get it since it is not original. However, then you have to live with the fate that the heater valve is much harder accessible for maintenance, through the fresh air opening, to say so. IMHO you can live with that since the heater system will certainly get attention now during restoration (cleaning, rust removal, greasing) while the system is out of the car.
Best,
Achim
(Magdeburg, Germany)