Germany 1929 Graf Zeppelin Balkan & Silesian Flight On Board Cancel Postcard

Postcard franked with 50pf Air Post, tied by Luftschiff / Graf Zeppelin c.d.s. (17.10), adjacent Breslau c.d.s. of same date, boxed rose flight handstamp at left, to Berlin, a scarce example of on-board cancelled mail from this less common flight.