so I have this NJS Dura-ace BB and on the non-drive side of it it has a rubber o-ring that goes between frame and the outer cup.
the BB was in the frame when I bought it, with the rubber o-ring.

I have never seen a o-ring on a BB like that before and seems kinda hokey.

any ideas?

is there suppose to be an O-ring there?

is this the octalink bottom bracket? for what it's worth, the one that i had came with the rubber ring as well.

which DA bottom bracket?
The 7710?

i have an isis bb with one of these and at first i thought it was a really crappy design as well. but it's not really.

if it's like mine, there's a lip on the adjustable cup that the tool mates with, rather than the splines found on most shimano bb's. because this lip does not bottom out against the frame (bb bottoms out against itself) the o ring simply covers the exposed threads on the adjustable cup outside of the bb shell. auspiciously protecting them from crap. i'm sure you can get away with not using the o ring, but it's there so why not.

once i got everything nice and tight (the shop really screwed it up) i've had no problems whatsoever with the bb.

edit: mine is similar to the 7700, not the 7710

Now I am very curious as the tech doc for the 7700 say nada about a rubber seal:
Installation

7610

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

yeah, on closer inspection mine isn't that similar to the 7700. mine is pretty similar to a un73, only it has the "special tool" (for me, park bbt-18) interface on both sides.

edit: just like that \/

7610 w/ "o" ring:

photo: John @ Businesscycles