diff interchangeability, Quaife/A4/4kq/540i?

[auditude at neta.com]

Hey all,

Check this out.  It looks like the Quaife part number for the rear A4
quattro diff is the same as the front 4kq part number.  As a matter
of the fact, the same part number is listed for a BMW 540i

http://www.quaifeamerica.com/differentials/diff_apps.htm

Does that mean these tranny's (and that diff) use the same exact
part?  If so, what swapping opportunities does that provide?  What
type of diff is the stock A4 rear?  The BMW tranny?

There must be more to the story, since the Bimmer Quaife's are on
sale, but the other two aren't.

Comments?  I realize I can't say I have traction problems, but I'm
curious about the options and interchangeability.

Isn't the tranny "the same" in the 5kq?

Thanks,

Ken