Broken hubs on this series are fairly common and every die made from that hub after the S broke would produce coins with that broken letter. (Each die would have made about 20,000 coins)
After 1868 the mint used the Master hub/master die/working hub/working die method of die production so there would be multiple working hubs. 1868 and earlier the working dies were made directly from the Master hub. That is why the 1868 rev of 68 is so interesting. The deterioration of the master hub can be followed from no broken letters through six broken letters before the Master hub was retired and new Hub of 1870 Master hub was created.