Casey,
Although I don't know much about edged weapons, this leather case does not appear to be any form of Civil War sword scabbard with which I am familiar.
A search on the NPS Soldiers and Sailors website shows no P.S. Elston as a Civil War soldier of record on either side, nor any Elston with the first initial of P for that matter.
Perhaps someone with more expertise in this realm than I can help you further.
Mike