Plumbing a radiator without a cap?

Hey guys, I got ahold of this used junkyard aluminium radiator for my dart to solve the cooling issues. Only thing is Im not sure how it should be installed.
It has no radiator cap, no problem I thought Ill just use an external one or an "inline filler neck" like pictured below. But whats with the hose? Its not an ATF cooler, there is only one opening.
Is this for some sort of pressure guage or is it for an overflow system? From what I understand the overflow hose should be "outside" the pressure release valve like in the second picture.
Any info would be great.