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.
The cathode is flat and whilst the control grid passes flat across the cathode the screen grid is less perfect in the winding - looks like lower tension and some kinks in the wire.
The classic envelope is 38 mm in diameter, and excluding the IO base pins, is 104 mm tall.
References: 1043 & 1040. Type KTW61 was first introduced in 1939.