6201RS

• 6 - type code: single-row, deep groove ball bearing
• 2 - series / robustness: (2 corresponds with ‘light’, which is more durable than 0, but less durable than 3 or 4)
• 01 - size code: 12mm bore (inner diameter)
• RS - grease seal: rubber seal one side (no seal would be ‘6201’, both sides sealed would be ‘6201-2RS’. Z and ZZ are used similarly to indicate presence of steel shield(s))

The 6201 is a good fit for this application, and providing the gerbil wasn’t too hard on it, should last a long time.