My 7600 DA BB has the same ring....I've had it there for years and had no problems....

My only guess is that BB was designed to work with both 70mm(english not italian threaded) and 68mm BB shells and the o-ring keeps the dirt out if you are using a 68mm shell..Since it's a cartridge BB, the cups are tight but it leaves a gap on the non-driveside. The o-ring fills the gap.It doesn't affect performance at all