Here is what I believe to be a light fitting from a naval vessel, possibly submarine. Unfortunately i have been unable to find any information or image that is similar.I understand that blue lights were used when ships/submarines were under action stations.It is of substantial construction the screw fit metal being heavy and over engineered. The glass is 4mm and has a base diameter of 6cm, with a height of 6.5cm. The retaining ring has an outer diameter of 6.5cm and a depth of 2cm. The overall length, from the end of the hooks to the top of the glass shade is 15.5cm. It weighs 580 grammes.