I'd just look at them, if they were both originally ET40 but the fronts have the inner lip of the barrels outside the centers and the rears have the inner lip of the barrels inside the centers that would explain the discrepancy between actual offset and the stampings.
I've never done this - only BBS wheels I've ever had have all been one piece - but it seems that this is occasionally done to change the offset and it doesn't seem that it's a real safety issue, as different offsets are often made with the same centers from BBS.
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