The final drive ratio on the petrol engined auto box may be different to that on the diesel engined auto box. It will be a lower ratio on the petrol one, which isn't good for performance or economy on a diesel car.
VX designate box as AF23/33, and it has many variants and final drive ratios.
http://www.opel-infos.de/getriebe/af23_33.html
It may be that a petrol one wouldn't fit a diesel one because things like the bell housing would be different due to things like the starter motor being in a different place.
This is why they use alphacodes, to make sure that the right box goes on the right car at the factory.
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