M3.Type:Submarine Monitor, Keel laid 08/12/1916, Launched: 9/10/1918,  Completed: 14/07/1920. Builder: Armstrong, Whitworth & Co, Yard: High Walker,  Yard Number: 911, 1600t disp (surface), 1,950disp submerged. Engines: 2 x 2400bhp oil engines, 2 x 1600hp electric motors, Engines, Possibly by Armstrong, Whitworth, Propulsion: twin screws, 9.5knots. Construction: Steel. Armament: 1 x 12.0ins, 1 x 3.0ins AA guns; 4 x 12.0ins bow torpedoes. History: 14/07/1920 British Royal Navy, 13/04/1932 Broken up. Comments: Laid down as a K class submarine, to be named K20. 1917: Renamed M3, Main gun loaded on the surface but fired when all but the muzzel submerged. 1928: Guns removed and converted to a minelayer. 1932: Arrived at J Cashmore, Newport for breaking up. 

Info courtesy of Tyne Built Ships (no copyright). RK