I don't know what the PO did before.... dump a can of oil on it and light it on fire is what it looks like. It was brittle as well and didn't want that stuff flaking down down into the pan. In far too bad of shape to try and clean up....
Surprisingly it mostly just sits in its entry point; there is a rubber gasket. One bolt holds it in place. Removal took a lot of twisting, I thought for sure something held it in there, likely just years of gunk/crud around opening.