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are you talking about the nut that goes onto those (2) studs?
if so, then it is 10MM nut that is found standard on many japanese imports.
you should be able to find it almost anywhere.... i know i got a handful of these laying around... if you really need it. PM me.. and i'll mail one to you FREE! i'll cover the stamp and envelope.
^ Yikes. I remember when doing that mod for myself and later on for a friend that it would be awfully easy to drop a nut down there (since the angle is so awkward). Looks like you'd have to practically tear down the car just to get to it.