This O'ring is for the Hayward CL200 series inline and offline chlorinators.  The O'ring is Made of Viton, a durable synthetic rubber resistant to pool chemicals.  Replacing this o-ring will often stop leaks from the chlorinator lid.
 
Part 5 in diagram
 
Dimensions 5-1/8" ID, 3/16"
Please apply marine lubricant before inserting