Disk Speed Indicator Mk XV for use with Norden Sight Grumman The outer rim has been

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

See instruction page from the SCR-595 manual in the photo gallery

Nakajima J1N Bomber Gekko aka "Irving"

The glass is clear and the lettering precise

Disk Speed Indicator Mk XV for use with Norden Sight Grumman The outer rim has beenThe 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