I'm not sure that this will help you, but...here goes nothing.
Oh, by the way, we asked PM customer service for the exact measurements and they did not know.
All that being said, when I saw the post card it was at the larger end in regard to size for US postcards. Per the USPS, the largest acceptable postcard measures 6"x 4.25" (~152mm x 108mm).
Aren't the European card measurements ~148mm x 105mm (5.8" x 4.1").
If so, you may be able to use you idea and force this "Liberty" card.