Most, but not all 9 inch Ford nodular cases were marked with a N directly above the pinion. The P cases are not 9 inch Ford cases, but '55-'64 Chevy cases. The P cases were made with one of the braces inside deleted so that the limited slip differential would fit in them. These were used mostly in Corvettes, but also in the high performance full size Chevys from '55 to '64.
Ford put the gear ratio, differential type and ring gear size on the tag. If you had a 9 inch Ford with a 3.25 gear and the Trac Lock differentail the tag would say 3L.25 9 xxx on the bottom line. If there was no L in the gear ratio then it was a standard differential. If it was a 8 inch rear end instead of the 9 inch then it would have 8 instead of 9.
Last edited by big gear head; 01-18-2013 at 08:56 AM.