Today in Postal History

Siam to Austria
November 17, 1920

This registered cover was sent from Bangkok.
The address side bears a preprinted registration label.

The cover was addressed to Graz about 150 km southwest of Vienna.
German Austria was used to distinguish the Austrian state which was created
from the German-speaking remains of the Austro-Hungarian Empire after World War I.*

There is a German receiving or auxiliary mark of some sort
starting with BRIEFPOST on the stamp side.
Can someone decipher it?

This is definitely baffling when you address the stamps.
The stamps, except for the 5s value, are not supposed to have been issued until 1921.
It may be that I'm misreading the Bangkok CDS date.

In any event, the stamps are from the 1920-1926 King Vajiravudh series:
2x2s brown on yellow paper; 2x3s green on green paper;
2x5s rose on pale rose; and a 10s black and orange.
(Scott 157, 188, 190, and 192).

There also seems to be a stamp missing from the upper right corner of the back.*

The cover looks suspiciously philatelic and the lack of transit marks
for a registered cover makes one wonder what was going on here.
Any suggestions?

*Thanks to Jim Whitford-Stark who found this quotation regarding the early days of
Austria after World War I in 1918, the link,  and also noted the evidence of the missing stamp: