kz1000 engine o-ring identification help

I have a Vesrah top end gasket set and it came with 2- 10mm o-rings , the black one goes to the tach guide but I can't figure out where the red one goes and it isn't listed in the kit or on the kawasaki parts fitch for the head or jugs, very puzzling and has me nervous about closing this thing up.


 

Could it be for the oil pressure sending unit? or maybe cam chain tensioner? 

Yeah, puzzling, I have convinced my self that it is nothing that I missed on the inside of the of the motor so no worries.
Almost done, should be rolling soon