Thank you all for your responses. Most helpful!
The tank is a solid plastic one and it does appear to be custom made for the boat as it is shaped to fit in the bow under the front deck.
That is a nice idea for outlets either side to counter any list but I guess if your that low on water you just need to fill up! All there are quite close and in one corner so it's not for that.
I was thinking of using one of the outlets for a sight gauge actually.
I like the logic of having the fittings all through the top. That makes a lot sense. But I'll stick with what I've got and plug off the two I'm not using.
Regarding the breather, is it possible that the filler pipe had a T junction and one branch functioned as the breather?
I guess I'll have to add one and there is a small skin fitting very close to the tank which would work to attach to.