The CV979 is a thermal delay switch based on the commercial DLS10. The specified delay time is between 60 and 120 Seconds.
The bi-metallic strip is the active element. Formed from two metals with differing coefficient's of expansion. When heated the greater expansion on the outside bends the strip over.
The heater is on the left, the bi-metallic strip in the centre and the fixed contact on the right.
The heater is two parallel lengths of coiled wire - probably tungsten.
The label on the plain card box. This gives the serial number as 5431 and the date as 1944. Admiralty valve specifications were called patterns.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 97 mm tall.
