I found one of these on each side when I started wire brushing my chassis today.

 

Could it be a sturdy surface to use a jack on?

 

It looks like a bump stop for the suspension to bottom out on.

 

It actually sits vertically, you can see the mounting holes just to the right of the box, looking at some of my other pictures it my be a bump stop for the front point on the crossmember arms, just ahead of the bush, but it would be metal on metal contact.