Fuel system help please!!

So I've swapped my fuel tank for a new one as I had rust in the old one. When I switched them I noticed that in the old one there looks to be a filter (maybe!) and nothing in the new one, just an empty square.

I am fairly confused here as obviously there is no way I can get inside the new tank to put whatever needs to be there inside.

Do I need to put an inline fuel filter in between the tank and fuel filter? Have I missed the point completely and it is something else all together? Is it simply good to go as it is?

I'm very confused and my project is on pause until I can figure this out!

Any help/advice would be greatly appreciated as I have been unable to find an answer online as yet.

Many thanks, Lee

Oh yeah, forgot to mention that my car is a 1983 1.8i GTI just incase that makes a difference.