Rossica has published a major exhibit on Russia and Persia which includes many pages of censored covers -- and not just Russian and Persian censors, but British, French, German and Turkish as well. Here are the links to the relevant frames:
http://www.rossica.org/RVG/myPh3/album/Russia%20and%20Persia/Frame%205
http://www.rossica.org/RVG/myPh3/album/Russia%20and%20Persia/Frame%207
http://www.rossica.org/RVG/myPh3/album/Russia%20and%20Persia/Frame%208
http://www.rossica.org/RVG/myPh3/album/Russia%20and%20Persia/Frame%209
Other sources include
http://www.iranicaonline.org/articles/philately-vi-postal-history
and
M. Amarloui, The History behind Censor Markings used in Persia 1910-1925, Oslo, 2009.