The 6NO120 is a thermal delay switch. From the application of the heater power the bi-metalic strip will heat sufficiently to bend and make contact with the static electrode in 120 seconds. The details of the device are encoded in the Type designation - 6 volt heater, Normally Open contacts and a delay of 120 seconds.
The main application is in providing a delay in the connection of anode volts to mercury vapour rectifiers - thus giving a suitable time for the rectifier to be hot enough to operate.
The heater site in the top outside slot in the ceramic pillar.
The outer fixed contact with the bi-metallic strip facing the heater block.
The Type designation is etched into the top of the bulb.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 68 mm tall.