As I understand Arrieta guns the 500 hundred series guns do not have certain features available on them such as H&H assisted opening system, the 500 series has a six pin lock while the higher numbered models have an eight pin lock, etc.
All of Arrieta's guns that I've seen are very good guns, I own a 570. The biggest difference in my opinion is going to be cosmetic. Some others here on this board who like to dabble in design/mechanical differences may be able to give you a more in depth description than I can.
The Arrieta catalog that I have from 2001 lists the 875 and 931 as its highest grade model.
Last edited by JM; 10/14/08 04:43 PM.