The Mullard MX123 is a Geiger-Muller tube with a very thin mica window. The main use is for the counting of alpha and beta particles. The inside of the tube is conducting and is the cathode. The anode is a central rod or wire - probably tungsten. The MX123 has a gas filling of neon, argon and halogen.
The central anode connection. The body is the cathode connection.
The protective end window cover.
The mica window showing the anode within.
A typical circuit configuration.
The wide metal tube envelope is 26 mm in diameter and, excluding the base pin, is 65 mm tall.