The standard hammer has a cutout for the DA sear. Some folks will remove this part to make the revolver function only in SA.
But, this modification does nothing to improve anything.
If you cock the hammer and look down into the action, you'll see the presence of this part or a cutout in the hammer from where the part was removed.
A true factory SA only K38 has a hammer that lacks the DA sear and the cutout to install the part. It is solid.
It's not difficult to spot, but some familiarity with S&W revolvers helps.
By the way, S&W used to sell a kit with a matched trigger and SA hammer for making the conversion legitimately.

I may be wrong on the following, as I never got the chance to make a real comparison, but my understanding is that the true SA only K38s come to full cock with slightly less arc of motion.
You supposedly get faster locktime and easier cocking effort.
Trigger pull is unaffected, and factory specifications are within ISSF rules.

And, not to complicate things too much, but from the 1930s to the 1950s, there were a special class of modified SA only revolvers (usually K38s or Colt OMTs) built for American Bullseye which featured heavily rebuilt actions, short throw hammers, "cockeyed" hammer spurs, etc. Here on the S&W Forum you can read more about these by searching for King's conversions, for starters.
They are rare and desirable for collectors and shooters. Not something you'll likely run into by accident, but they do fit the discussion.

Jim