This is the HSR-1425CR Continuous Rotation Robotic Servo from Hitec.

• 360 degree continuous rotation
• Dual ball bearing equipped motor

• HSR-1425CR Robotic Servo with Hitec/JR/Z connector 0.94" (24mm) diameter round horn four gromments, four eyelets and four 2x15mm mounting screws

• Installing in robot and connecting to receiver and linkage

• Operating Voltage: 4.8 - 6.0V
• Operating Speed: 43.4 RPM +10%
• Output Torque: 38.88 oz-in (2.8 kg-cm)
• Weight: 0.33oz (41.7g)
• Size: 1.59 x 0.77 x 1.44" (40.6 x 19.8 x 36.6mm)