its a fabricated 9 inch ford housing... "fab9" and it accepts: well 9 inch components... that being said, you can just as easily stuff a 12 bolt diff and ring/pinion in there if you use a strange engineering 12 bolt drop out centersection, just be sure to order the axles with 9 inch ford axleends....
the advantages o fthis is the reduced drag and weight of that unit...