It is either part of the body just behind the rim holding the glass since it has the bolt and wing nut, or it is part of the ring that held the glass which would have had a hinge on one side and a bolt and wing nut on the other side to hold it shut. I guess it depends on whether the bolt and wingnut were part of the body, or the ring that held the glass. I am guessing that this is probably part of the hinged ring that held the glass. This is a very neat industrial archeology find, whichever it is.