Welcome to the new and improved Proflex / K2 Tech Forum!
I don't know about the clunk or whatever in the shock. Sometimes the are "sticky" if not ridden in a while...BTW do not lube or whatever to the shock shaft. Set the sag and giver her a go!ooops I should read before I post....Without spring on the shock turn the rebound knob all the way one way. Then compress the shaft and see what the rebound speed is. slow? or fast? Turn it all the way the other way and it should be fast? or slow? No difference or no rebound at all means shock service. Most have it in between if you are doing just XC stuff. fast rebound is great for washboard type surface, slow rebound is better for dips. Hope this helps.
I have a Nr2 that does exactly the same. It could be that the seal has gone a bit hard or the oil in the shock gets a bit sticky when the nitrogen has gone. A re gas with nitrogen may cure it or its probably a rebuild with new seals. Worth trying a re gas first as these are old shocks that have been sitting round for a few years.
Essentially it's a "9-Style" bike, also known as the '97 "World Cup" frame.but as for the specific model. it could be a 956, 857. 957, or a 3000.