Armorer 951 hit the salient points, and also provided some nice pics of the current revised extractor design. Yes, the lower edge, as clearly seen in the pics, is rounded.
I don't have any pics stored of unused early 3rd gen extractors, but I did have this pic of an early one (bottom/broken) and a current revised one in stainless (top). The raised rectangular surface behind the inside of the hook is the adjustment pad, which is filed to bring the hook into proper tolerance with the breech face.
The GO/NO-GO gages used to be a bar block (checks reach of hook edge to opposite breech face shoulder) and a flag gage (checked dimension between rear of hook and breech face behind it). They stopped providing the flag gage when they decided proper use of the bar gage, and proper filing of the adjustment pad (flat, not angled) was enough to determine when proper spec was reached.
The extractor spring tension can be different when looking at the old and new style breech faces (rounded machined shoulders with narrow cut at top, versus straight wide cut). Also, they used to offer a 'repair' extractor spring set for the early 3rd gen double stack 9's that had a nested set of springs. The reason given was that some of the older hand-controlled equipment might have made the spring hole with rounded bottom edges that wouldn't let the normal spring sit flat in the hole, but a lighter outer spring was supplemented with a taller 'inner' spring.
Also, we were told that sometimes an older gun needing that nested repair spring combo might need to have a higher tension than normal, but it was critical to make sure the feeding & extraction were still within normal spec. I came across a handful of early 5903's that required repair in that manner and ended up with heavier tensions, but ran just fine.
The nested repair spring set was eventually made 'obsolete' and replaced with a pair of different tension single springs an armorer could try, one of which would be more likely to work in any particular gun, instead of trying trial and error by trying all the optional springs available.
Extractors seem to run for quite a while in normal conditions, barring abuse like dropping a round into the chamber and letting the slide run forward, forcing the extractor hook to slam against the case rim and snap out and over it.
Extractor springs can become weakened over time. It can also matter at which end of the normal tension tolerance range an extractor spring might've been when installed, too, since there's an acceptable lower/upper tension range for different calibers/models that armorers check with the Wagner force dial gauge. If it was at the lighter end of the range to begin with, it's not surprising it might become too weak, sooner, than if it had been at the upper end of the tension range.
I usually replace the extractor spring if replacing & fitting a new extractor, but if it's just a weak spring, I won't replace the extractor unless it's damaged.
In guns with the early style ejector, I started replacing the ejectors (with the new style) when replacing extractors. The new style ejectors gave faster ejection and a corner had been rounded that might sometimes lend itself to a stress riser (and maybe eventually a broken tip).
If a local gunsmith is familiar with fitting extractors and checking extractor tensions on S&W pistols, and has the tools, it's preferable to just trying to 'drop in' an extractor and spring (especially in an older gun) and hoping the right dimension and tension occur.
The factory still does repairs, but you'd have to call and ask if they'd do it under warranty (they might ask if you're the original owner), and it might take a while. I think the TDA guns are shipped from Springfield to the Houlton plant, and the techs there are also the ones doing repairs on .22's, PPK's and SW1911's, last I heard.