The question is not was this a genuine lettercard, but how could it have been used?
In one of the pages, Secret Writing, there is a document that you can download, it was released in 2011 by the CIA. It talks of microscopic writing and how, using orange colored ink, an agent could write a secret message across the stamp and it would not be visible to the naked eye unless you used a ruby colored lens. This had the effect of making the stamp invisible and the writing clear.
This same document also tells us about the use of any lines found on a document, within them it states, you could 'hide' Morse code' or in the Somerton Man case it could be micro writing hidden within lines or flourishes or even larger letters. There were and are people around who could with the naked eye, write letters as small as .1 mm in height. Look up James W. Zaharee for more on this.
Here's a question for you, on the example lettercard above and if you were a spy, where could you write and hide secret messages?
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