Today in Postal History

This cover was sent from Rome to Vienna.
It was carried on board the Graf Zeppelin to Friedrichshafen,
the Zeppelin's home base, and then forwarded to Vienna via regular mail.

There are four special Rome Zeppelin CDS.
There is also a Zeppelin machine slogan cancel on the reverse.

The front received a nice stamped cachet for the
flight based on the Romulus and Remus story.
The design was taken from the current Italian definitives.

The cover's arrival in Friedrichshafen on Bodensee was
marked by a special postmark on the reverse dated May 30.

This particular flight was the occasion for special stamp issues by Italy,
San Marino, Greece, Aegean Islands, Cyrenaica, and Tripolitania.
This cover bears the complete set from Italy.
There are 3 l, 5 l, 10 l, 12 l, 15 l, and 20 l (Scott C42-C47).
Their value as a used set is quite nice.

The cover also has a a 2.75 l from the 1932 10th anniversary
of the Fascist government and the march on Rome issue (Scott 304).

There is a stamped serial number on the front.
I don't know whether covers on this flight were all serialized or not.
Help, anyone?