Oring is placed between lens and the scope housing to remove any vibration of the lens at the time of the shot. Actual size is 2.0 inches to fit the 1-3/4 HHA Scope.
HHA make many different models. The size of an HHA scope is measured by the inside diameter of the scope tube. Please know the correct size scope tube when ordering. 
HHA 2" takes 2.20" lens and o-ring
HHA 1-3/4" takes 2.0" lens and o-ring
HHA 1-5/8" takes 1.75" lens and o-ring
HHA 1-3/8" takes 1.50" lens and o-ring