There are 3 lines on my Merc 40 fuel pump. Fuel in,out. What is the 3rd one for ?

[Mr. Merc 40]

The 3rd one goes to cowling. It is screwed down and has an odd plug or "nipple". There is a hole in the end. What does it do ? Thanks..

[boats.net]

What is serial number and model of your Mercury?

[Mr. Merc 40]

Serial 6142138 Model 1040522
Believe it is for remote mounting a pulse pump. Should it be solidly plugged? Is this hole required ?

[boats.net]

That is water cooled fuel pump, should have fuel line from filter, fuel line to carb, water line from block and water line to tell-tale fitting.