The KT24 was issued as a replacement for the KT21 output valve. The KT21 had a steep slope (gm=5.3 mA/V) (KT21 data-sheet attached).
We believe that like the KT2 it is is a 'critical-distance' tetrode, based on the Harries patent. This type of tetrode requires a large anode diameter and is efficient only in relatively low-power applications.
With an anode load of 10,000 Ohms, a single valve would produce 650 mW output from battery supplies.
The Osram label.
The top mica: two filament tension springs can be seen but not the grids.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 90 mm tall.
References: Datasheet & 1043. Type KT24 was first introduced in 1939.