Drivetrain How to identify year of rear end?

F0ZW-xx just tells you what it was originally engineered for, a 1990 Mustang. That engineering number prefix (F0 = 1990) gets carried forward every year, and possibly on multiple models, until it get's a major update. It doesn't change every year so it does not tell you it is from a 1990 Mustang.

What you need is the casting date code that is near that engineering number prefix and suffix, or on the other side of the differential housing. (I can't remember where the casrting date code is on a diff housing.) The casting date code looks like a plate with a screw on either end, and has three to four letters and numbers.