This is the Futaba S9157 High Torque Servo.

FEATURES: Ideal for large competition airplanes
Coreless motor
Dual ball bearings and metal gears
Metal middle case helps dissipate heat for cooler operation
One year limited warranty

INCLUDES: Futaba S9157 High Torque Servo with 0.83" (21mm) diameter round horn,
1.8" (45mm) diameter round horn, 1.2" (31.5mm) 6 point horn, 1.5"
(38.5mm) 4 point horn and mounting hardware

REQUIRES: Installing in model, connecting linkage and connecting to receiver

SPECS:    Torque: 425oz-in (30.6kg/cm) @ 6V
Speed: 0.14 @ 6V
Size: 1.6 x 0.8 x 1.5" (41 x 21 x 37mm)
Weight 2.5oz (71g)
Horn Screw Size: 3x8mm

COMMENTS: Use with NiCD batteries only
A note on Coreless motors: Conventional servo motors have a steel
core armature wrapped with wire that spins inside the magnets. In
a Coreless design, the armature uses a thin wire mesh that forms a
cup that spins around the outside of the magnet eliminating the
heavy steel core. This design results in smoother operation and
faster response time.