I'm a bit confused. Is there a reason that a mk 3 requires different cocker spaniel restraints to whatever you used in your mk2 ? (If any

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I am not being entirely facetious. I understand the magic seats arrangement in the mk 4 differs from the mk 3 so maybe the mk3 differs from the mk2 The rear head restraints are more intrusive. This may, or may not, make them more cocker spaniel proof.
I had a cocker spaniel many years ago. She got very excited at first on every car journey until she settled down. I think short journeys require as much safety restraint as long journeys, if not more.