There is information in the FAQ about identifying brake components including how to decipher the code on the VIN plate which says which brand were originally fitted front and rear.
By this stage in their life-cycle previous owners or mechanics could have swapped almost anything in by now.
I think 40mm rears were most common on the late model estates. If replacing both at the same time that is what I would probably try to go for. If only replacing one then it is more import to match the opposite side to avoid introducing an imbalance. 40mm piston will give about 10% more stopping power than 38mm for the same hydraulic pressure as the force relates to the surface area of the piston.
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