Paul,
These plates appear to be new or near new. The cc is vivid and there is a varnish protecting them. I see no marks indicating they were even installed on a gun. But that's not to say they weren't installed, why else would they have been made and engraved.
I don't know what gage gun they would be off of. I don't know if there is a difference in size between 20 ga and 12 ga plates.
I know some of the late Ithaca assembled guns were a mixture of parts that didn't follow typical grade designations. Maybe the gun you saw was one of them.
Chris