That's an '89-'90 year RS250 rebound adjuster.
You need to refer to the RS250 manual to see how to set it up. The dampers are different to the NSR set-up.
Did you get the forks from Paul Tye by any chance?
If you did, they are my old forks from my race bike. A mixture of NSR stanchions and lengthened damper rods/tubes.