The KTW61 is a variable μ RF amplifier pentode, and was first introduced in 1939. This tubular version is probably of post war manufacture.
The MWT paper logo.
Above the pinch is a lower screen that hides the grids.
The open helix of the suppressor grid with the close wound screen grid within. At full size the control grid is clearly seen.
The plain box used for valves supplied to the UK Government.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 101 mm tall.
References: Data-sheet, 1040 & 1043. Thanks to Anurag aggarwal for sending the Osram data-sheet and Peter Bates for the box. Type KTW61 was first introduced in 1939. See also1939 adverts.