There's another thread on Wildings etc, but I can't find one on this subject so here's a card I was delighted to pick up at the weekend.
The picture side is nothing special, but the 4d 1964 Geographical Congress paying the postcard rate to Belgium 20 days after the date of issue, with a nice clear postmark and reasonably tidy writing.... in Australia a contemporary item would rate highly. Not a bad pick-up for less than £1.

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Ian Billings -
Norvic Philatelics GB stamps info:
http://www.norphil.co.uk - also /catalog for our ecommerce site, blog.norphil.co.uk, shop.norphil.co.uk and Ian_norvic on twitter