The DES1 is a thermal delay switch. Typical uses of these devices was to prevent the application of HT until the cathodes of protected valves were at the correct temperature. The Mercury vapour rectifier in particular is sensitive to the early application of anode volts.
The construction is of a heater warming a bi-metallic strip. This strip bends as it heats and eventually will touch the fixed contact and make the external circuit.
The envelope is frosted but the bi-metallic strip can be seen.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 96 mm tall.
Updated January 18, 2013.