Aloha Paul,
You are correct, that heater is used in TR3's. It is mounted so the doors open downward and the fan is controlled by a rheostat/off switch mounted on the dash. The two round outlets are used to mount flexible ducts to supply warm air to two defroster nozzles. There is a valve mounted on the cylinder head to control the flow of hot engine collant to the heater. There is no remote control for this valve on a TR3, so it must be set with the car stopped and the bonnet up. The operating pressure of the TR3's cooling system is 4 PSI. These heaters are prone to leak because of age, I recommend you have it test for leaks at pressure before installing it.