Here is a rather intersting First Flight Cover - Not rare but some interesting transit markings on the reverse.

You can see it has been addressed to General Delivery in New York City and the back markings tell the story of a rather convoluted route with 4 offices handling it (5 if you count the Montreal Cancel). Albany on Oct.1 then Central Sta., NY on Oct. 2, New York, New York on Oct. 2 and NY General Delivery on Oct. 2. Also the two different slogan Cancels applied.
