This is a part of an ash pit. The Ash Pit is the chamber/support for the barrel, and traditionally houses the ash pit grate {where you riddle the ashes} in solid fuel cookers.
We have seen cast behave like this before, but only in 'snug burner' pressure oil conversions. And then it was the barrel. You do see it too in the barrel with simple singular 13amp element systems that do not regulate temperature well. {another reason to convert!}