The KTW61 is a variable μ amplifier pentode for use in RF or IF stages within an AGC controlled superhet receiver. The control grid connected to the top cap reinforces this view. The literature gives the device as a pentode but the KT prefix from M-OV indicates a beam tetrode design and beam plates are present rather than a third wire grid.
Below the top screen one can see the beam plate.
A closer look at the top of the electrodes. The control grid has copper supports and the secondary emission suppression is by the use of the beam plates - seen as bright plates just outside the screen grid supports.
The classic envelope is 38 mm in diameter, and excluding the IO base pins, is 98 mm tall.
References: 1043 & 1040. Type KTW61 was first introduced in 1939. See also1939 adverts.