I purchased at an auction a Camillus fixed blade knife that I need information about. It is a fixed blade that looks like the U.S.M.C knife but it is marked U.S.N. On one side of the blade it is marked "Camillus, N.Y." and on the other "U.S.N". The sheath is marked "U.S.N". What model do I have? Is it a real military issue or a civilian knife with military markings? Also about what time period was it produced? Any information would be greatly appreciated!