[Nimby]

I have a Keihin K F carb on a gy6. I took it apart (my first time) to clean it. I followed a really good you tube video and had no problems getting it apart and understanding what I was doing. I cleaned it really well with carb/choke cleaner and blew it out with air. I ordered a re-build kit and I am waiting on that. Heres the problem, I picked the carb body up yesterday and a tiny black O ring fell out. It is only 3/16 OD. Real tiny. I have no idea where it came from. I looked up some parts diagrams on the net and I cant seem to figure out where it came from. There is a small O ring on the drain screw so I have that. Could it be from the low speed jet? When I took out the low speed jet there was no O ring on it. It has to be from something that could be removed and still leave the O ring in place. Any ideas? Why cant I see it on the parts diagrams I found?