I believe it was made by my great uncle Walter Furnish, most likely in the trenches during the Battle of the Somme in World War One. The handle is a .303 shell and the blade is probably spare metal from the battlefield. This kind of thing is known as trench art because they were made by soldiers in the trenches or prisoner of war camps. It's been in the family for years. Walter died in the 1970s and the letter opener was passed on by his wife.