Today in Postal History

This airmail cover was sent from Riga to Berlin.
Delivery was planned to be poste restante
(held at post office for patron's visit - general delivery).
There are four attractive airmail CDS from Riga.

The nice triangular airmail semipostal stamps are a complete set
of the 1931 issue to collect funds for the Latvian Home Guards.
The stamps were made by overprinting airmails originally issued in 1928.
There is a 10 santim deep green with a 50 santim surcharge,
a 15 santim red with a 1 lat (100 Santims) surcharge,
and a 25 santim deep blue with a 1.50 lat surcharge (Scott CB3-5).
The airplane on the stamp is a Bleriot XI.

The cover arrived in the Berlin Central Airport Post Office on May 21.
it received both a CDS receiver and a special round cachet.

I'm not sure what the blue cross on the back indicated.
I doubt if it indicated registry.
Any suggestion?