The KTW61 is a variable μ RF amplifier beam tetrode, and was first introduced in 1939. The literature classifies these valves as pentodes but the images show the bright beam plates.
The up arrow for an RAF valve.
The top screen and electrode supports passing through the top mica support.
Thanks to Anurag aggarwal for sending the Osram data-sheet.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 105 mm tall.
References: Data-sheet, 1040 & 1043. Type KTW61 was first introduced in 1939. See also1939 adverts.