The CV342 is the UK Government procurement specification for the commercial VLS631 thermal delay switch.
The heater will take a defined time to heat the bi-metallic strip to the point that the contacts will make. Such switches were used to delay the HT voltages until the heaters of the valves were at the correct temperatures. Mercury vapour rectifiers and traveling wave tubes are examples of devices that are protected by thermal delay switches.
This implementation of the specification is quite different to CV342 but the delay time will be just the same.
A closer view of the white glass bead and the thick heater wire.
The moving contact complete with heater
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
Reference: Datasheet. Type CV342 was first introduced in 1946.