StreetkidThe Purist models do not have a fourth GMT hand. The Purist models can track a second time zone using the rotating bezel, while the GMT models can also track a third time zone with the fourth (red) hand.
With that said, the Purist models are more traditional... but I went with the "Mystery GMT".
StreetkidFrom what I have read, the watch movements are the same. My guess is that the Purest models simply don't use the normal 12 hour movement (that is used for the hour hand in the GMT models)... but instead attach the hour hand to the 24 hour movement (that would normally be used by the GMT hand).
Some have suggested setting the red GMT hand to the local time, so it will sweep the local 24 hour time. Thus for 6 PM / 1800 hours, the GMT hand points to 18 at the traditional 9 o'clock position while the regular hour hand points down to the 12 at the traditional 6 o'clock position... thus always being able to tell the correct hour, night or day.
I work in the aviation field, so for me the GMT hand will be set to GMT/UTC/Zulu.