I had the luck to spend some time with the late Giovanni Metelli, head of the Beretta premium gun department. His view was that each generation of SO guns was an improvement, contraty to the common view that older is better.

The SO design is the only high end OU I have encountered that manages to incorporate side reinforcement, in the form of the replaceable shoulders, that keeps the action narrow as well as shallow. Compare the width of an SO with any "Boss" design to see the difference. It is one of the best OU sidelocks ever built.