This is a set of O.S. Engines Mixture Control Valve O-ring's, intended for use with the #6H and #7D OS carburetors.
Specifications: 
Outer diameter: 6mm 
Inner diameter: 3mm 
Thickness: 1mm 
 NOTE: These O-Rings can also be used for other applications where a 3x6mm silicone o-ring is needed.