Was there ever a documented occasion where fanning the hammer on a single action revolver was employed in a gunfight by anyone in the past?
I doubt it. And not because we can confidently say it wasn't done, or was never done, but because the "documentation" from the wild west days is simply either nonexistent, or unreliable.
First off, valid accounts from those days rarely concerned themselves with the nuances of
technique.
Second, even when/if they did, it was common practice to ..embellish.. "stories".
SO, if you do find "documented" evidence, take it was a grain of salt, it may be true, or it may be "proven" by eyewitness accounts given to the likes of Ned Buntline....
One of my favorite westerns is Eastwood's UNFORGIVEN. Not because of the action, or most of the plot, but because of the way the characters "debunk" the popular western mythos about guns and gunfighting.
They make a point of how the fastest draw and the fastest shot often were the LOSERS in the gunfight.
and in the real world, wasn't it Hardin, who was shot in the head, and the coroner couldn't tell if he was shot from in front, or behind?? The statement about how "if he was shot from the front, it was an excellent shot, if shot from behind, it was excellent
judgment" comes to mind....
Now, Slip hammering is a technique, and does get results, from those practiced in its use. Elmer Keith had a couple of SA set up for that, and they had lowered hammer spurs.
Fanning a Ruger Blackhawk? something you will likely only do ONCE!!
You CAN build a SA for fanning, but fanning a stock SA is good way to break something, quite rapidly.
The real Old West was not nearly as entertaining as movie and TV westerns.