How do I find out which rear differential I have fitted?

Hi,

I have a 2004 Forester STi (Japanese import) and the pinion bearing on the rear diff is on it's way out.

I'd like to get a replacement diff before removing this one so I need to identify which one is fitted. I looked underneath today and the only markings I can see on the bottom of the diff are "03 Y" and "KX 43". Neither of which seem to help!

Any help would be appreciated.

Thanks

Dave.