Disk Speed Indicator Mk XV for use with Norden Sight Grumman See instruction page from the

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

The glass is clear and the lettering precise

Nakajima J1N Bomber Gekko aka "Irving"

Type D-1 (also AN5816-2)

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 See instruction page from theThe 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