Disk Speed Indicator Mk XV for use with Norden Sight Grumman The glass is clear and

The glass is clear and the lettering precise

Six mounted in in various places

Nakajima J1N Bomber Gekko aka "Irving"

The outer rim has been painted red and white to denote a preferred operating range

and likely used in other bombers such as the Vickers Wellington and Short Stirling

Disk Speed Indicator Mk XV for use with Norden Sight Grumman The glass is clear andThe Disk Speed Indicator Mk XV was used by WWII US Army Air Force and Navy bombardiers to verify and calibrate the disk speed of the Norden Bomb Sight during flight before initiating the bomb run. The disk speed is the parameter which determined the duration of flight between sighting of the target and automatic release of the aircraft's ordnance. With the bomb sight switched on, the bombardier would affix the top axle to the small female receptacle