Why do the patents on google patents contain 's1' after the patent number
e.g., http://www.google.com/patents/USD678656
USPTO doesn't seem to have S1: http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=D678656.PN.&OS=PN/D678656&RS=PN/D678656