The term 1st model, 2nd model and 3rd model comes from collectors as a way of differentiating the various changes in the Dragoon as the manufacturing process progressed, it was not used when the revolver was in production.
The Dragoon in the photo would be considered a 3rd model because of the round trigger guard. It may be an early or late 3rd model, the type of latch on the loading lever would determine that.
I see mykeal beat me to the draw, ha ha