Well you have a bit of a conglomeration then, the Norco valve is not used with the Recirculating heater. A lot can happen in 67 years, motor failure, valve failure who knows? A full Air-Flow installation looks like this from the cab.
https://www.flickr.com/photos/advance-design_parts_co/17058542759/in/album-72157651718017649/
The reason you do not have the plate is it was most likely an Air-Flow install. This is the picture of the two block-off plates provided with the Recirculating heater.
https://www.flickr.com/photos/advance-design_parts_co/15838946822/in/album-72157631868452754/