This is the HS-503HD Ultra Nano Digital Aircraft Servo from Hitec.
FEATURES: Metal Karbonite gear train
Ideal for micro 3D, sport aircraft and small helicopters
Fully programmable digital circuit
Three pole motor
INCLUDES: HS-35HD Ultra Nano Servo with two 1.1" (28mm) double arms, one 2.1"
(54mm) double arm, one 1.2" (31mm) double arm, one 0.67" (17mm)
four point arm and three mounting screws
REQUIRES: Installing in aircraft and connecting linkages
SPECS: Operating Speed: 0.10 sec @60°
Output Torque: 11oz.in (0.8kg.cm)
Weight: 0.16oz (4.5g)
Size: 0.73 x 0.30 x 0.61" (18.6 x 7.6 x 15.5mm)
COMMENTS: Operating voltage should not exceed 4.8V.