Through hulls on Sagawind 20

This year we bought an old sail boat from 1979 - a Sagawind 20. My first order of concern is to check the location and state of the through hulls, which are perhaps the most critical part of a sail boat. There is literally a hole through the hull!

Special valves - called sea cocks - keep water from flowing in and the state of these is important as the boat will sink if they leak.

Through hull locations

The boat was in the water when I bought it, but the previous owner has sent me some pictures from when it was on land. I've tried to spot the through hulls and numbered them below.

It looks like 1 and 2 are below the water line, while 3, 4 and 5 are above the water line. I believe that (?) is just some gritty stuff and not an actual through hull, but I will see when I get it on land.

Seacocks

I've found three seacocks inside the boat, two for the toilet (probably drain to holes 1 and 2) and one seacock for the sink. The wierd thing is that I don't see a through hull where the sink's seacock is located. If the sink drains to 3, then why is there a sea cock when it's above the water line? This leads me to suspect that there might be one more below-water-line through hull, that I have not found in the pictures. I will see when the boat is on land the next time.

Here are some images of the seacocks. I've been told that the previous-previous owner changed them, but I'm a bit uncertain about the material used. I will try to diagnose the next time I'm on the boat. My fear is that he chose brass seacocks, which will corrode very quickly. We will see. If that is the case, I'll probably spend the money to exchange them to bronze or plastic (Marelon) seacocks.

Here is a very nice description of different types of seacocks: https://trip.ayy.fi/wordpress/wp-content/uploads/2014/08/Seacocks.pdf