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.
I recently took apart my weed whacker and it used the same ball bearing. I actually learned that these ball bearing casings (?) are super common. I replaced mine with a skate board bearing and it's as good as new.
Its a mass produced bearing type. Most likely the cheapest option for both devices. Optimizing a bearing for a gerbil wheel seems like the real overengineering in my opinion.
This is one of the counter intuitive things about production at scale. I remember I added factory cruise control to a Mazda 3 by buying the steering wheel with the buttons and having a technician flip a switch in the ECU. Everything else was already there.
They never made any additional changes to non equipped cara because it's cheaper to just produce 1 part or 1 configuration and digitally lock it out. 2500$ option with a 200$ fix.
Thing is, when I had gerbils, their exercise wheel turned on a cylinder of plastic inside another cylinder of plastic. I also had one that was a wire wheel with sheet metal spokes and a wire base, the "bearings" were the wires of the base poked into holes in the spokes. A gerbil wheel having a ball bearing at all seems a little bit extra to me.
Not all bearings are created equal. This might last for years. This also might explode into fast-moving metal shards in a day. It shouldn't be a big problem if it does, as an angle grinder should be built to contain exploding bearings, but it might be worth ordering a higher-spec bearing to have on hand before you need it in a hurry.
Those look like skateboard bearings to me, they are quite common.
A relatively high quality set of 8 bearings is like 15 bucks if you have a skateshop nearby