The MKT4/5 was a beam tetrode version of the earlier (1932) MPT4 output pentode and was intended as a plug-in replacementfor the discontinued MPT4. . The beam tetrode version had the advantage of a lower distortion allayed to a higher output.
There is a B7 based variant having an identical electrode cage but of course a differing valve base - tant pis!
In single ended audio configuration could provide an output of 2.5 Watts.