The G10H is a side window Geiger-Muller tube designed to detect ionising radiation. The tube has a low pressure gas filling and an incoming particle will cause some ionisation of the gas. The ionised gas conducts and a pulse is produced across the tube. These pulses are counted and a rate indicates the strength of the radiation.
Rob has pointed out that the PET60 is virtually identical but has a working voltage of 410 Volts.
20Th Century Electronics. A UK company based at New Addington, Croydon, Greater London.
The end window with a spacer holding the central electrode. The foil inside the tube is the second electrode - cathode.
The thin glass tube envelope is 24 mm in diameter, and excluding the B4 base pins is 183 mm tall.