Rather than comparing date of first registration, given the number of '04 9s sitting in dealers for a few months, would it be a good idea to compare frame numbers. I would guess the frame number will be allocated at the point of build and therefore if it is a dodgy batch of master cylinders they should be fitted around the same time.
From the frame numbers, if they are within say 200 frames of each other, then they were probably built at roughly the same time.
Sorry to hear of the problems...sounds like an issue that needs quantified as soon as possible !
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