Hmmm...could be a plug indeed. The brass is primitively cast. I doubt it's earlier than mid-19th and possibly much newer. If 19th, it could be American, Confederate CW, as they were known for crude, one-off pieces (like trench pikes, boot knives, etc). Could be Span colonial or Mexican, again for crudeness. Don't think it's European. Last guess, African?? Those blunt quillons were popular on French/Amer knives ca 1860's. Note some naval dirks had them, but this is NOT a naval piece.
-Looks too business-like to be a simple letter opener or tourist-
