I checked my MP today and this is the result:
MP part number is MP-1-270-10050059.
The bearing is a 6302RS; 42mm diameter, 15mm hole, 13mm width.
You might also find it listed as 6302-2RS or 6302RS because it has a rubber seal on each side.
The 6302-ZZ is basically the same bearing but with metal shields instead of the rubber seals.
These metal shielded bearings usually produce slightly less frictional drag than the rubber seal versions, but they will not prevent water from getting into the bearing when you wash off your wheel.
Alan P.S. I forgot to mention that this sized bearing is often used as a front wheel bearing on motorcycles too, so it might be worth seeing if your local motorbike shop keeps them in stock, but don't forget to check the prices first.