AOPA NAVAID with original envelope. Complete instructions.  Two discs which rotate around a common center.  Outer scale of the larger disc is a 360-protractor similar to the compass rose printed on navigation charts.  The inner scale (yellow) of the larger disc is referred to as the MILES scale of the computer.  The top scale of the inner disc is referred to as the MINUTES scale of the computer. 4.25" diameter.  Very good vintage condition. Original owner; smoke free home.