Can I use a SRAM PC850 chain on a 6-speed bike?
You may want to contact them to see if they have changed their design. I am currently using a PG 850 on a six speed. I seem to remember when I bought it, that it was labeled as a 6,7,8 speed and at the time the 9 speed was unique. Perhaps they have made changes to  make it a more popular 7,8,9.