It looks kinda looks like the little o-ring seal that goes on the sprocket shaft right behind the seal that goes in the case. I'm 100% the picture isn't the best, plus there is only one that goes there.
OH MY GOD, dont you crank that engine up.
Those two C looking things, were they in a white & clear little plastic bag. If thats what they are, then its the wrist pin cer clips for sure.
Please tell me you havent cranked the engine yet.
Those clips got to be in place or else the wrist pin will move in the piston & then chew up the cylinder wall.
I hope your pulling our leg, cause if not, that cylinder has to come back off to install those clips.
ofcourse i'm kidding! i had 2 rubber o ring type things laying around. my buddy gave me one, he found it laying on the floor when he was machining my cases, said he didn't know if it was from the cases or not. then i found another one laying on the cardboard where i tore the engine apart, this one was new looking? they weren't part of my new sealed gasket set and have no idea where it came from...
Sapsucker, :mellow: skeered me half to deffis, most near did gib me a heartastroke.
I could just see him running the engine like that & killing that cylinder & after I spent 3 days in porting it up. Boy that would hurt my feelings.