One thing that was visually different on the upgrade when I sent back my cracked slide was a beefed up extractor. FWIW, I am much happier with the changes ,whatever they were, from a performance point of view. It has been flawless but many here have experienced the same good consistency since 2004 without any upgrades. It seems that there were some issues in the 500-800 S/N range that have been worked out since that time. This was also about the same time frame as the move to Deer Park, which may or not be related.