Not easy to find information about this stamp but it seems to be related with the first voyage of the Graf Zeppelin to Brazil in 1930.
Sites are in Portuguese:
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http://www.oselo.com.br/newsdesk_in...4b1a7354b0ab-
http://pt.wikipedia.org/wiki/Selos_..._-_1900-1939As described in the first link, a set of stamps were ordered specifically for this voyage from Germany to be used for correspondence sent on the Zeppelin. The stamp pictured above is Z-6, an overprinted Condor stamp with the 5000 réis (5$000).
The stamp may well be this one:
23 de maio de 1930 - Selo Aéreo Zeppelin 5000 Réis Verde (Graf Zeppelin Usa)
May 23, 1930 - Air Mail Zeppelin stamp 5000 reis green (Graf Zeppelin USA)
A rare postal envelope is in the first link:
E4673-1930-ENVELOPE RIO DE JANEIRO/USA with the rair Z-6
Sent May 24th, 1930 to Cincinnati, Ohio.
Hope this helps!
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