The KTW61 is a variable μ RF amplifier beam tetrode, and was first introduced in 1939.
The Osram brand label on this M-OV valve.
The top screen. The control grid connects to the top cap by a length of tape. The inner two grids can be seen to have notched supports. The third support is plain as it holds the beam plate in position.
Thanks to Anurag aggarwal for sending the Osram data-sheet.
The classic envelope is 37 mm in diameter and, excluding the IO base pins, is 101 mm tall.
References: Data-sheet, 1040 & 1043. Type KTW61 was first introduced in 1939. See also1939 adverts.