What is this?

We picked this up at a local flea market but do not know what it is for sure and niether did the vender. It appears to be a 9 x 12 cake pan 2'' deep with a lid attached that can not be removed, welded fast by manufacture but slighly curved on only the covered end? This is however quite a conversation piece in our collection when company comes. We have gotten all kinds of comments on this piece. We collect this color of graniteware however so we purchased it ( the price was good!) We would very much like to know what it is. Does anyone know what this pan may have been used for?