Part Identification Please on Mercury 135 Elpto 1999 Sn og694327

[Ceilidhalfie]

Hi there. I have just joined this forum and am looking for some help. When de-winterising the engine I noticed that a metal part has split in two and broken off. It looks like a water hose connection to the right cylinder head ?? Looking at the spark plugs, it sits between the middle and bottom spark plugs and has a screw head about the same size as the plugs. It looks like its connected to a water (?) hose. There is also a small pipe coming out of the side but nothing seems to have been attached to this. Inside the part there was a plastic piece and spring. I cannot find any description of this part which is attached to the right cylinder head cover.
Any help and info would be much appreciated.
Thanks.

[boats.net]

Maybe it's the thermostat housing, V-135 DFI (2.5L) Mercury Outboard 0G590000 THRU 0G760299 Cylinder Head Diagram and Parts