The M8163 is a special quality voltage stabiliser.
The tube strikes at 180 Volts and regulates at 150 Volts. For output currents of between 5 mA and 15 mA the voltage regulation is 5 Volts.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 57 mm tall.
Reference: 1040. Type M8163 was first introduced in 1955.