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 fixed contact is on the left. The lower right has the heater enclosed in a ceramic cylinder. The geometry of the cylinder and the heater power determine the time delay.
The moving contact face view with the outer support frame and top mica.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
Reference: Data-sheet. Type VLS631 was first introduced in 1946.