I am sorry I have misplaced this thread... in the wrong subforum. I was doubt to place it either in the World covers or USA covers... and picked up by mistake the one which is not connected at all :/ A MODERATOR can freely relocate it in the subforum where it will better fit.
My thought that sender(s) just wanted to do their job seriously, and paid the extra Registered. Possibly in 1936. in USA people have been already alerted to Hitler's German, and Zeppelin possibly have had problems with missing their regular mails?
There are a lot of theories of conspiracy about zeppelin Hindenburg being sabotaged... maybe Zeppeling company in that time have found out that lot of their mails don't arrive on the addresses when they have been sent... or that have been been opened on the way (the Registered mustn't been opened any way).
Possibly that is a reason why this leaflets have been so scarce?
@ Hoxsie454 - dates are on the back of the cover:
