Changing Lower Unit Fluid in F300?

[F300User]

Sorry for my first post to be a seemingly simple question. But here goes:

I can't figure out how to access the lower unit drain plug on a 4.2l F300. I presume the drain is behind the water pick-up screens. And on the port side there appears to be a hex-type of screw set well into the screen. But no tool that I have matches that screw. Anyone know what tool is necessary?

Thanks.

[boats.net]

You will need a 4MM allen wrench to remove screw on water inlet screens to gain access to gearcase drain screw.