take out that rubber piece, no oil will come out.  there should be a threaded hole there.  and the outside screw should have 2 flats for a knob.

heres some pictures of the adjustment knobs on the 500.  last pic is the 2 parts new from suzuki, knob and screw, and the part numbers.   

87 shock has 2 knobs, one on the rezzie and one on the shock.  88-90 shocks have a slot for the adjuster on the shock, and a knob on the rezzie.