The MPT4 is an output pentode made with an external anode. The design is known as a catkin - M-OV CAT valves were large cooled anode transmitting valves such as the CAM3 audio modulator.
One of the curious ironies of the MPT4 saga is that the catkin version of 1933 although a 'blind alley', was technically very superior to the glass version MPT4. Comparison of the two photos makes it quite clear that the large anode used in the glass version could not possibly be used in the catkin version and shows the benefit of the external anode design.
The two versions are, in fact, entirely different designs. The compact catkin version was possible because the external anode was directly air cooled and did not reflect excessive heat back to the cathode-grid region of the valve.
It took M-OV about five years to learn how to make large glass power pentodes with grid structures which did not emit electrons, or buckle, due to overheating.
