What's the difference between the 3 series and 4 series posi units? Is there a part number on the unit to identify?
Thanks.

10 bolt 8.2" ED32118 2 2.73:1 & down
10 bolt 8.2" EDB30116 3 3.08:1 & up

10 bolt 8.5" 410409N 2 2.56:1 & down
10 bolt 8.5" 410408N 3 2.73:1 & up

12 bolt ED32088 2 2.73:1 & down
12 bolt 30140PM1 3 3.08:1 to 3.73:1
12 bolt EDB30174 4 3.90:1 & up

Or by measurement:
http://i531.photobucket.com/albums/dd358/sfatzo/carrier.jpg

http://thumb7.webshots.net/t/59/659/3/15/26/2890315260041712107VGzqQV_th.jpg (http://rides.webshots.com/photo/2890315260041712107VGzqQV)
2 series in front on the left, 3 series in the center and 4 series in the rear right. Notice the difference in the location of the ring gear flange. Click on the picture for a larger view.

I've got a 3.07 ratio 68 12 bolt posi coded CD, which series carrier might this be?

3