This Is An SDI Gauge (Surface Deflection Indicator) from Robart.
An important tool used to measure the amount of up and down or left and right on your model aircraft elevator, ailerons, flaps, anD rudder.
Precise measurement of surface deflection in INCHES.
Measurement is in 1/16th increments, with 1/4,1/2,3/4 & 1 in. marked. Fits large and small scale model aircraft.
No more errors with rulers and scales. Quick and easy surface adjustments. Allows for accurate differential adjustments. Instructions are provided.