I just looked it up, the 5205 is a Double Row Angular Contact bearing,... Bore 25mm, OD 52 mm, width 20.6 mm.
It has a static load rating of 3,280 lbs, vs 1,750 lbs for a single row (6205)
Bore and OD are the same, width is wider on the 5205 to accomodate the second row of balls.
It was likely spec'd with this as the torque on the sprocket could tear up a single row in a hurry, especially with a floating rubber carrier.
I would get a rubber seal on both sides, if available.
The "C" numbers with respect the the 6000 series of bearings, indicates internal clearance differences. They assort them at the factory as they are measured, as some industry will demand certain clearance.
A C3 is catagorized as "standard" clearance. That number would not be important for this application.