From Colt's point of view there was only one Dragoon revolver, which they called a "holster pistol," (as in saddle holster). The first, second and third descriptions were created by collectors to describe substantial design or internal changes. On Colt's part, they factored in these changes or improvements as they went along, without any particular comment.
The rectangular bolt cuts in the cylinder made it possible to simplify the cylinder bolt by eliminating a machining operation, and provided additional surface in the notch for the bolt to lock against. The notches were machined, not ground.
After 1860 the Dragoons became the “old model holster pistol,” where the 1860 Army became the “new model holster pistol.”