What's the deal with these parts?

I was pulling out my gas tank and came across these bracket-things that BARELY covered the top edge of the tank, bolting into the trunk floor. Also, they just popped out, as there were no nuts securing them underneath. Were they supposed to just thread into the floor sheet metal? What purpose do they serve, and where can I find new ones/part # since these are too corroded to put back in.

Much thanks.

Edit: from a '65 fastback

Trailer hitch... that's interesting. Thanks.

So were those holes custom drilled for the hitch or stock locations by Ford on all stangs?