1974 Merc 20hp where's the lower unit vent screw?

[BrucePHenry]

So, I'm looking at this old thing and I see two screws. One is about the diameter of my pinky, black, and appears to be plastic. The other is a fairly small, like #6-ish, pan head screw.

Which one of those suckers is the vent screw? And if it's the larger one, what the heck is that small one?

 

[MTboatguy]

Vent screw is the small pan head screw above the cavitation plate and the drain and fill screw is on the bottom of the lower unit in the bottom of the bullet nose, it is about the same size as that black screw you show in your picture. It is located where it starts to round up.