Ronald Trist & Co. Ltd.

The company supplied "Mobrey Switches" (a water level switch - these were connected to the engine stop solenoid so in the event of low water the associated DMU engine would be shut down) that was used in many types of DMUs.

The illustration is a trade advert from 1960.

For a company history see Grace's Guide, a company employee lapel pin has made it into the National Air and Space Museum Collection at the Smithsonian!