Between the 551 and the 550 there isn't a huge difference. Aesthetically there are nearly the same. The biggest thing are different blade steel 551 is Elmax, 550 is S35VN. There scales are a different pattern. The only thing I can remember, ( someone correct me if I am wrong ) I thought Thomas mentioned a different process for cutting the lockbar on the 0550 which I think was due to 0551's sometimes having lock rock. Again I could be wrong.