The Tillotson HR High-Speed Mixture Screw Washer fits between your fuel adjustment needle spring and O-Ring. 

The washer prevents the spring from crushing and tearing the O-Ring that seals the needle and the fuel high or low fuel circuit.

This particular brass washer fits only on the high side needle of the HR Carburetor. Do not try to use this on any other needle. Always check your carburetor listing before ordering. 

78A-245 OE Tillotson HR High-Speed Mixture Screw Washer fits All HR Model Tillotson Carburetors