Here is a wartime airmail cover from occupied Rotterdam, Holland to Paterson N.J., USA. Cover is postmarked 16 October, 1941. Large violet rubber stamp indicates transatlantic airmail service from Lisbon to New York. Cover is franked with reprinted gull definitives which were overprinted to show their value and obliterate the country name "Nederland."
German censor markings on the back include violet boxed censor #, and violet handstamps of the Wehrmacht tying censor sealing tape. The sealing tape bears the code letter "e", which at this time signifies Nancy, France...a branch of the Frankfurt censor center which handled mail going to southern France, Switzerland, and North and South America.
Source: Mail Surveillance Under the Third Reich
by R. J. Houston.