The red wire is the +12 volts, the white wire is the - or ground 12 volts. The two blue wires are the ones to the thermostat.
I don't know where the thermostat was on your '75, 31'. Apparently it is missing? Look for holes somewhere about chest level which may have been the location. Inside wall, like on a closet. You may see wires in the closet that went to it and if missing, they may be just cut off or have tape or something on the ends. They may not be blue on the thermostat end though, not sure what AS used for color, it might vary.
Best guess for now.