I just bought a Pietta 1851 Navy. The grip frame is similar to the original Colt 1860 Army, not the original Colt 1851 Navy. The original Colt 1860 Army had a downward rear slant insofar as the grip frame was concerned.
Keep in mind there was never such a thing as a ".44 Navy" that ever left the Colt factory in the 19th century. It is a modern invention.