Part #: 6579 (Associated Diff Drive Rings) : This item has been discontinued

[XLR8]

They made the new case (the new version of the 6565 and uses the old 6565 number) narrower between the bearings so you can't close the case if you try to install a vintage diff. You have to use the modern diff outdrives along with the case To quote my old instructor at Ford, they could mess up a wet dream

I haven't checked but I think it's safe to assume FanRC outdrives are patterned after the vintage part so they won't fit the new case either.
Bottom line: don't buy a new AE case unless you have new AE outdrives to use with it.
FWIW, I have both sets and I paid $22 for AE and $30 for FanRC.

The outdrives are the highest cost transmission component. Maybe this is an attempt by AE to disrupt the flow of counterfeit parts.