After a bearing number, you may see additional letters. These letters are: S, Z, SS, and ZZ . What these indicate is the time of shielding is on the bearings to prevent contamination of the bearing. The S stands for rubber shielding and the Z stands for metal shielding. SS and ZZ mean the shielding is on both sides, not just one.
You can learn more about bearing prefix and suffix numbers here: http://www.engineerstudent.co.uk/bearing_numbers_explained.html