Ok … this has to be a RARE one …. A replacement piece of porthole glass! I am sure there are stories it could tell! This could have been kept onboard in the “maintenance’ supply area to be used if there was a porthole that needed replacing.. but it sure looks like it could have been installed at one time as well. This baby is HEAVY .. and parts of the edges are “chipped” .. they almost look scalloped on purpose. The SSUS had pretty small portholes .. I am sure they were Navy standard for safety! There can't be too many of these around! This may require special packaging and shipping and I dare say will be too heavy and fragile to ship internationally.