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 soft glass envelope has become detached from the hard glass button holding the pins. There seems to never have been a proper glass-glass bond and there are signs of adhesive around the base of the tube.
The glass button is fabricated from a short length of hard glass tube that is softened and pressed around the middle of the tri-part base pins.
The surface of the base glass shows no sign of having been melted into the envelope.
The smooth surface of the detached envelope.
The heater passes through the ceramic tube and this introduces thermal inertial and thus slows down the heating of the bi-metallic strip moving contact.
Both parts of the switch have hard contact metals joined to the supports.
From the rear of the fixed contact.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
Reference: Data-sheet. Type VLS631 was first introduced in 1946.