I found pictures of a used mk4 Seat Leon bumper for sale on eBay - see below. Not a
Cupra Leon bumper, but I dare say the manner in which the number plate holder is attached to the bumper would be the same. You can see in the picture that there are four holes (circled) where the fixings for the number plate holder pass through the bumper cover. They are quite widely spaced, so if anyone were to remove the number plate holder and fit a smaller, shorter number plate, then those holes wouldn’t be covered by the number plate. It would be necessary to get the holes filled and that section of the bumper (or the whole bumper?) repainted.
The number plate holder on the mk7.5 VW Golf is attached in the same way. A guy on one of the Golf forums recently posted a picture of his car with a short number plate fitted and the plate holder removed, and the fixing holes are visible.
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Reverse side;
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