how do you tell the difference between series 2 and series 3 posi units?

 

what factory gears are instaleld on it or the thickness of the flange

 

2 series on the left, 3 series on the right.

Look at the location of the ring gear flange. Note how the one on the left is almost flat on the back side (to the botom of the pic) toward the bearing; where the one on the right is more cone-shaped. That's because a 2 series pinion gear is larger diameter than a 3 series pinion gear.