If it was the hammer or rebound slide frame stud, last I heard (in my revolver armorer class) the newer ones are press fit. They can be replaced, unless the frame has been damaged, and an alloy frame might be more easily damaged than a steel frame. Fortunately, it's quite uncommon to come across a damaged one (unless someone's been tinkering inside, or may have used the wrong rebound slide spring which put excessive pressure against the slide's stud, etc).
I currently own 7 J-frames, 5 of which have alloy frames (Airweights and a pair of M&P 340's with Scandium aluminum frames).
The 2 steel frame guns (649 & 36) date from the 80's and the rest are from more recent production. The older production 649 needed the attention of our former revolver armorer in order to get it to function when it was NIB, but that was from a general time period when S&W could be a bit of a hit or miss affair in QC.
Since the only realistic & practical way to become skilled with a 5-shot snub .38 Spl/.357 is to train & practice with it, that means most of mine have seen a fair amount of use.
The least used has been a 37-2 DAO, which only sees standard pressure loads fired in it, and which I only bought because I'd always wanted to own a M37. The factory DAO variation (bobbed hammer) seemed like a nice one to pick up. It has the pre-Magnum frame and metallurgy, and after some discussions with the factory I'm only using standard pressure loads when I use it. I'd rather not risk cracking the frame under the barrel, as may sometimes happen in some of the older alloy guns with hot loads. Why punish a nicely done older style Airweight, anyway?
I'm trying my best to wear out at least one of the 642-1's and the first of the M&P 340's (from the first production run, with the ILS/lock), though, and the older steel guns are still in great condition and spec.
I've always wanted one of the +P rated 638's, myself. My older 649 has been a fine snub over the years (since being repaired), but it's heavier than my Airweights and heavier than I'd prefer for a pocket holstered gun. (Although not as heavy as my 3" Heavy barrel M36, or my SP101 DAO.) I think the alloy +P rated version of the 638 would be nice to add to the collection as a counter point to the 649 ... even though I prefer DA/DAO trigger modes for practical training/practice and usage. The SA is nice for occasional slow-fire target/precision shots while practicing at longer distances, to keep the basic skillset from rusting away.
FWIW, as a revolver armorer who sees a fair number of J-frames brought through our range as off-duty & secondary/backup weapons, it's mostly the older style (pre-MIM) guns that have been brought to me for repair or correction. Aside from cleaning out some really grungy newer style snubs (cops carry them where they gather a lot of debris, or else are less-than-charitable in their cleaning practices
), the new style MIM guns have been giving some pretty good service.
As a matter of fact, one of our guys just brought me a nice nickel M36 from the middle 80's with a problem. The cylinder stopped turning. Broken hand torsion spring. I didn't find the loose broken end until after I'd removed all the excess goo in the frame.
Enjoy the 638 when it arrives.
FWIW, it's not a necessarily a bad idea to own a second copy of a favorite handgun, especially one that may be carried as a dedicated defensive weapon, if you can afford it.
For one thing, it helps make having one repaired less of an inconvenience.
For another, if one is ever taken into evidence/property after being used in a shooting incident, it may be out of your possession for some time and the second one might be handy.
Lastly, some folks who buy 2 of their favorite (or commonly used) handguns may like to keep one for heavy range/training use, and one that's "lesser used" for carry. Personally, when I've bought "duplicates" of handguns, I've generally just used the extra ones to share the wear & tear of my frequent shooting.
