We are in the process of building a kitchen on an overgrown lot that had a restaurant / bar on it until the building collapsed in 2010-ish. We have been working on clearing the old slab so the grease trap can be inspected and we found this...thing. It is metal (steel I think) and quite securely fastened to the slab. We tried to pry it up with the rake and shovel and it does not move at all. The one side may be hinged, but we're not sure. It is about 3 ft long and stands 4inches high. We will be back out there this weekend and I am going to take some water and a scrub brush to see if it has any markings. In the meantime - any ideas?