Rust near my rear shackle mount, what do you think?

So I found some picture online of what that mounting box is supposed to look like, very helpful as I had no point of reference to go on otherwise. As expected, it was just a slab-o-steel someone had welded on, not even that well, and it seems all it did was trap moisture and rust things out.

Cut it off and now I have a clear idea of my next move. I'm less worried about hitting this as a new a guy new to welding since it's not an exterior panel.

I guess someone was trying to reinforce it or something? It's ben mentioned it was possibly left over from some trailer hitch setup, who knows. Any other guesses why this would have been there?