None that I'm aware of.
Anyway, did a bit of googling. Early RS6 rad. S6/A6 4.2 rad. They look pretty damn similar to me.
BUT!
1) Even if physical dimensions and fittings are 100% the same (and condenser mounting tabs, etc), they still might have different cores & different cooling capacity.
2) Late RS6 rad is different in that it has an extra fitting for aftercooling pump. You can get away with just plugging that, but potentially it can cook your turbos eventually.
Aftercooling systems are pretty different between early/late RS6 - they have different directions of (after)coolant flow (and late models have an extra electromagnetic valve).
Is your rad FUBAR? Or, if it's repairable, might be a best idea to just go that way.