6" (medium) Allen Hatch. Includes hatch cover and base but no "O" ring. Also supplied with an Allen Hatch Cover Bag
The gasket (base) is fixed to the hull of the boat and the A.538 O ring is placed around the gasket, under the small lip. The hatch cover lid is then screwed tight onto the gasket and forms a seal with the O ring around the external diameter of the hatch.