This is a correctly franked cover posted from the Centilux philatelic exhibition on the last day of the show, June 4, 1952. The Centilux tête-bêche pair and the 2 F Europa commemorative pay the 4-franc UPU letter rate and 4-franc registry fee. The oval backstamp documents the cover's arrival at Isleworth, Middlesex, England, June 7, 1952. And so it's a nice modern postal history cover.
But it's a lot more. As a young collector in the 1960s, I enjoyed putting together a modest Liechtenstein collection. In those days, long before the advent of Internet technology, obtaining philatelic information and solving philatelic mysteries required dedicated correspondence with knowledgeable senior collectors. One of the kindest and most helpful was the late Jack Beken, the British gentlemen and Liechtenstein philately authority to whom this cover is addressed. No matter how naïve or elementary the question, Jack Beken always provided a satisfying answer. It was a sad day in 1977 when I learned that this distinguished philatelist had died.
I sold most of my Liechtenstein material long ago to finance Luxembourg purchases, but the eight-stamp block below remains. When I acquired the block for $6.00 from a Matthew Bennett auction in the 1960s, I was clueless why it was imperforate and ungummed. Of course Jack Beken had the answer -- it's a proof!
Now we have ever fewer devoted philatelic authorities like Jack Beken. When there is none, philately will be remembered mostly as a curious pursuit rather than the learned endeavor it is.