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Prodrive say the P1 is built to spec, not from an STI5 but they must have got the parts from a parts bin somewhere. Looking at release dates it's more likely to be an STI6 but hey, who's counting.
They've all got the same bits in the end. Just out of interest, will the dealer sell you a P1 gearbox without a chasis number or os it the same as the P1 body parts?