The Stock Reference Number in your instruction book is F653 & F654

These fittings are handed left & right the outer edge is rounded at the bottom the inner is square with a triangular wing that goes behind the glass the fittings will have been supplied with the greenhouse from new.

Because they need to be handed they are supplied as Blank Ends and are sealed across the Outlet when you decide which end of the greenhouse you would like the water to drain you simply knock or cut out the membrane before you fix the pipe [ the pipe is not supplied ] and the fitting will now be the Outlet.

They are made to fit Europa Manor Greenhouses which where mainly sold through Hombase Stores. 

The Models where