93 Camaro Radiator Question

The radiator in my girlfriends Camaro is a total wreck; it's leaking from multiple places and needs to be replaced. I've never worked on the car before and know nothing about it; still not too big of a deal, except that
there is a whole below the cap facing the engine a little bigger round than my thumb; it looks notched on one side (towards the center of the car). Long story short it is open to the world and I have no idea what belongs there, does anyone know?
Thanks in advance for any help
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