There were NO 9 inch rears installed by FOMOCO into 69 Falcons I am told. To check your car........if you can put a socket on the bottom center section bolts.....it is a 8 incher.
This has been mentioned many times here in previous post. It is a old junk yard trick.
This is a 9 inch, 31 spline, Detroit Locker with 3.50 gears.
Some folks believe you can tell spline count by looking at the ends of the axle where the brake drums go. I have read this also on the Web. I have found it is not 100 percent accurate.