lol, yeah I think I could figure out what the groove is for if it was there... there's no groove on his. It's just threads, then a flat spot right up to the lip. No matter what I tried the O-ring won't go inside the housing. It just smashes up against the edge then pops out. But anyway... I solved the problem. Maybe in the future I can find him a cap from a different year that has the groove and will fit his.