Anyone knows generations of B&W Nautilus?

I have interest of B&W Nautilus speakers, the flagship produced since 1997 according to Bluebook.

It is said there are 4 generations of Nautilus.
Anyone here could help supply more details about differentials between generations?

1st - Nautilus
2nd - Signature (better x-over, exotic finishes and woofer response, limited release)
3rd - 'S' (regular finishes, similar x-over and woofer response to Signature series)
4th - Diamond (All Diamond tweeter, new finishes)

That is my guess on the generations.