On a local forum in my area a guy is trying to sell a 6" 66-1.

He doesn't seem to know much about S & Ws however he is reporting the hammer will occasionally CATCH at about the halfway point almost like a half cock of a SA revolver.

Several of us informed him this is not normal but what could be a possible reason, or cause for this? Thanks!

Likely suffering from what's called "push off". Due to wear and tear or owner induced damage to the bevel on the trigger, or the cocking notch on the hammer. Could be something as simple as a modified (cut) or aftermarket, very light rebound spring in the gun.
When suffering from push off, the DA sear in the hammer can often catch on the trigger as it falls.

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Ret. LE, FA Instr, S&W Armorer