The Red O-Ring is designed to seal the threaded lid on all of the fermenters. The O-Ring is FDA approved for food. The diameter is 4 inches. It fits in the slot specially designed for Minibrew® so it will not fall out as you put it in place to screw into the threads of the fermenter.
The O-Ring is standard equipment on all fermenter orders.
The second picture show the top of the f6.5xe gasket with the 1/2 inch Stainless Steel Weldless EZ Seal installed.