This is a vintage Roller-Smith D039 Radio Altitude Indicator that was used by the U.S. Navy during WWII. Meter is in great original condition. On/off switch seems to work and you can change range from hundreds of feet to thousands of feet. Back data plate reads 1D-14B/APN-1 110 QB/47 NXsa-56848. Thanks for browsing. WE ARE THE MUDLARKS, we find treasures so you don't have too!