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: