Some background I found out for the forge; It was purchased from a school county auction in western part of Virginia early to mid 90's. The high school had an Ag Shop class where students would learn blacksmithing and welding - a family member remembers using forge while taking the class. We believe school was using it from the 60's to the 90's.
It does appear to be concrete in the pan. Not sure but there appears to be an electric motor underneath as well - could the school have used an alternative fuel source at some point and filled in the pan to support whatever the students were working on?