The nautilus 803 came out in 1997. The newer 803S came out I think in late 2004. The "S" series was a big upgrade over the older nautilus series in my opinion.
I am speaking from the perspective of the 805. B&W had the nautilus 805 at the same time they had the nautilus 803. They came the signature 805 which was a big upgrade over the nautilus. They 805S came after the signature 805, didn't offer the nicer finish of the signature 805, but sonically was much closer.
You'll notice that the 803S goes for more money. The 803D uses the diamond tweeter, the 803S does not.
I am speaking from the perspective of the 805. B&W had the nautilus 805 at the same time they had the nautilus 803. They came the signature 805 which was a big upgrade over the nautilus. They 805S came after the signature 805, didn't offer the nicer finish of the signature 805, but sonically was much closer.
You'll notice that the 803S goes for more money. The 803D uses the diamond tweeter, the 803S does not.