Spot the number on the Snap On print and that is where the part goes. There is a small recess where this o'ring fits on the #18 rear end plate. You will never get all of these parts on a regular Snap on tune up from their service centers.

Brand: Unbranded
Non-Domestic Product: No
Type: Air Tool Parts
Warranty: No
Custom Bundle: Yes
Manufacturer Part Number: PT338, PT350
Bundle Listing: Yes
Country/Region of Manufacture: United States
Modified Item: Yes