[John Mackie]

The vast majority of deep grove ball bearings use METRIC sizes.

the last 2 digits of the indicate the bore size.

04 equals 20 mm bore
05 equals 25 mm bore
07 equals 35 mm bore and so on up to 99

60 62 63 and 64 are comonly used to replace older series numbers

62 series is the most common series.
60 and 618 are smaller section bearings , smaller balls.
63 and 64 series bearings are bigger, heavier.

6203 is the comon generator / alternator bearing 17 x 40 x12 mm.
6207 is a comon front gearbox bearing 35 x 72 x 17.

Z or 2Z = shields.
N = circlip grove.

there are pages of other suffixes .
I hope this helps,