This uses an original equipment vacuum solenoid from some other make and model which unlike the MGOC valve needs to be powered to apply vacuum to the carbs.
The relay operates and releases with the ignition switch, i.e. is operated all the time the ignition is on to run the engine. When the ignition switch is turned from the Run position to the Accessories position the relay releases, and power from the Accessories position is connected through the relays normally closed contact to operate the solenoid. When the ignition switch is turned to the Off position power is disconnected from the relay and hence the solenoid is released. You do have to remember to pause the ignition switch in the Accessories position until the engine has stopped.
Note: On 71 to 74 models there is an inline fuse connecting the Accessories white/green from the ignition switch to the green/pink for the heater fan, wipers and electric screen washer, use this green/pink for the relay contact. On models before and after this there is no factory fuse so it would be advisable to fit one when connecting to the white/green.