The S207/3K is clearly a thermal delay switch. It provides delays from 13 to 24 seconds and can switch up to 500 Volts at 200 mA - typical values for substantial, equipment type, mercury vapour rectifiers.
The type of heating is given as: an active b-metal strip heated by radiation from an incandescent heater. The switch also incorporates a device to compensate for variations in ambient temperature.
The maker is given as ITT but made in England and thus this was made by STC.
Behind the lettering on the main image can be seen the coiled heater.
The heater on the left and the normally open contacts with the fixed contact on the right.
Additional maker's codes.
The getter holder at the top. The ceramic cylinder is probably part of the compensation function.
A closer view.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.