Mine came from the factory with the modified seal - to my knowledge it's been done twice and it hasn't completely stopped the leak 
I did buy a replacement tail shaft but there was nothing wrong with it and it doesn't interact with the seal so can't be to blame for the ongoing leak.
I think the stupidly precise measurement of how far in it should be is probably the route cause. Easy enough to achieve with the box out of the car, but near enough impossible with it in!
There's recent threads on this where part numbers are posted - there's even a flange advertised for sale on here that's BNIB (not mine)
I would strongly recommend checking the flange before buying a replacement. No point buying it if it's on the car already!
The 'old style' seal is part number 018 409 399b
The one for the modified tail shaft is 01v 409 399a
The modified flange is 01v 409 809a and RRP is about £190
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===SOLD===2002 Audi S8 D2 Final Edition (yes, really) in Avus Silver with purple extended leather===SOLD===
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