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 travelling wave tubes are examples of devices that are protected by thermal delay switches.
The heater is in the centre of the ceramic tube. The tube forma a central part of the thermal mass and thus the time it takes for the bi-metallic strip to heat and bend.
The side view showing the fixed contact on the left. The bigger the gap the longer it takes for the moving contact to bend.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
Reference: Data-sheet. Type CV342 was first introduced in 1946. See also1946 adverts.