Unknown Spacer/O-Ring

[Rantz]

I had my entire engine dis assembled to replace my starter clutch (We all know what that entails) And I happen to have this spacer and O-Ring left over after putting my case back together, I checked everything as I went but I want to see if anyone can tell me what they might go to, I referenced them next to a US Quarter for comparison. Thank you to anyone that can help. So close to riding this thing finally and I don't want to forget something important and ruin an engine.