The QQV02-6 is the smallest of the double beam tetrodes in this 1950s series from Mullard. It was introduced for 470 MHz (UHF) mobile transmitters.
It is used in frequency multipliers and driver stages, but for small output powers can be used alone as a final power amplifier.
The thin glass tube envelope is 19 mm in diameter and, excluding the B9A base pins, is 57 mm tall.
References: Datasheet & 1040. Type QQV02-6 was first introduced in 1958.