There's some better descriptions and discussion of the X-Metal generations in this thread:
http://o-review.com/forum_detail.php?ID=11871
But overall, the traditional generations ended by 2001 or so, giving the last few years of the serials a fairly stable base. The only other change that happened after that was the switch to Hydrophobic lenses, the new coins, and in turn new SKU numbers. Functionally there were no corrections despite the flex couples still posing issues, but they didn't fix that, and instead just opted for a different approach with the X-Squared.