Are you sure that is a wear indent? That is the most perfect wear I've ever seen. Looks factory? The other pic is two blurry to see anything. I don't think it is a wear. But if it is, it's too much to fill in with welding. Too much heat.
I think it's a factory nest for the bracket or shims. It looks forged into the frame rail.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn