When I bought this card it was the lovely yellow Karmann Ghia that caught my eye, then it was that Vauxhall Cresta PA on the left that piqued my interest. It looks a little out-of-place in Cosmopolitan Ostend despite the American company, but then I realised that it was undoubtedly an Opel (though I’ve never seen one) and they were probably quite common at the time.
This card was posted from Belgium to a Mr & Mrs George in Surbiton in 1967. The writer’s in Ostend for the day and describes it as ‘really grand’. Such a quaint expression.
You were right first time – it’s a Vauxhall PA. Opels of the period were called Kapitan and had their own (uglier?) styling.
http://en.wikipedia.org/wiki/Opel_Kapit%C3%A4n#mediaviewer/File:Opel_kapitan_1950s.jpg
The next Kapitan was much squarer like a Merc, so it is not that one either.