Allen Hatch Covers are made from a lightweight polypropylene. Available in Grey, Black and White.
A.137 and A.237 are a non O ring design and have exposed fixings.
A.337, A.537 and A.637 all require a rubber O Ring to keep them watertight.
A1737, A1837 and A1937 feature a fully threaded cover with watertight seal.