Robert,
The 9" has been used since August 1956, by FoMoCo and ran production as standard rear axle assy. in passenger cars thru 1964. After 1964 it became a specialty rear axle assy in limited prod for SVO, HO and other performance vehicles. It also was used in F Series trucks from Aug 1956 thru end of 1979 production run.
Okay that being said, what's in each rear depends on how vehicle it was in was spec'd when built. As a general rule passenger cars & light trucks tended to use the 28 spline rear, although the bolt patterns for truck & car are diferent, trux being the larger, 5 on 5.5" pattern.
Only real way to know what any 9" FoMoCo Rear is, or has in it, is tag on one of bolts holding 3rd member in. OEM is @ 2:00 o'clock when facing pinion flange. If Tag is elsewhere somebody has been in rear axle assy at some point.
Posi is a GM term, FoMoCo has 3 different traction device rears, Limited slip, Traction Lok, or Detroit Locker. the limited slip is FoMoCo & uses cones. The Traction Lock is FoMoCo's version of a clutch type traction devoce rear, similar to GMs Posi, and Detroit Lockers are outsourced units that FoMoCo installed for various reasons. Ford also has a 4 pinion "Open Type" differential.
The 4 pinion 9" rear has 2 extra pinion gears, or spider gears inside the differential case instead of the commonly found 2 pinion set up. Ideal 9" rear for street use, in my opinion is a 31 spline, 4 pinion, open type rear axle assy. I run one under my 66 F 100 & it has a built 460 that sees 12.600 ET's @ 124+ mph, consistently. I merely back up a wheel rotation or more & drive straight into the box. When trans brake kicks off both wheels hook & I leave. No power loss thru a traction device or any thing.
Way to find 9" you want is get a parts identifier & start looking for it. They're out there if you know what you are looking for. I just bought another 31 spline, 4 pinion, 3.25:1 9" for $125. Here's something else you might like to know. For Sure, after 73, any F100 LWB or F150 with a 352 CID or larger displacement V8 came with a 31 spline 9" rear. Trailer & Camper spcl's had 3" x 11" rear drum brakes too. . . . FBp
