The DLS16 is bi-metallic strip delay switch. After the pre-set delay the moving contact touches the fixed contact and HT is applied to the protected circuit, usually a mercury vapour rectifier.
See alsoDLS15 for the same switch but with a 4 Volt heater.
Robert says that this valve has been fitted with a new base as the original spigot was broken off.
The heater is a vertical coil passing through the U shaped bi-metallic strip. The gap ensures a delay in the radiant heat from the filament raising the temperature of the strip.
The fixed electrode is on the left. At full size the join between the two metals forming the moving electrode can be seen. The moving contact is welded onto the moving strip.
The wide glass tube envelope is 32 mm in diameter and, excluding the IO base pins, is 61 mm tall.
References: Data-sheet & 4049. Type DLS16 was first introduced in 1949. See also1949 adverts.