SL-C9 Pulley Gear
This is part of the pulley/threading gear assembly from the SL-C9 / SLO-1700 / SL-F1.
Tape loading and ejecting is noisy or fails completely.
This part often fails although the machines eject function is often not effected. The cog on the underside of the gear often splits resulting in noisy operation.

Early production models used a pulley gear with an integral metal bush. These suffered from a design fault in that the metal bush would expand causing the outer plastic ring of teeth to split. Replacement gears supplied by Sony no longer have this metal bush.