The KD61 is a miniature neon voltage stabiliser with flying leads. The tube will strike at 85 Volts max and for low currents the regulation is sub one Volt. This is designed as a reference voltage supply in a stabilised power supply rather than to feed the HT voltage of a circuit such as an oscillator.
The anode is denoted by the red paint.
The evacuation pip.
The thin glass tube envelope is 12 mm in diameter, and excluding the base wires is 42 mm tall.
References: Datasheet. Type KD61 was first introduced in 1956.