Elaborating on the theme of Polish "Offices in Danzig" stamp issues...
Bee See uploaded a very informative note from Scott's Monthly Journal February 1925. In the note was a report of an appeal by the Danzig Senate to the International Tribunal requesting that they reaffirm that Poland only had the right to supervise the mail at the Polish Post office. Meanwhile, (Polish) Postal boxes had been put up all over town for mail being sent to Poland.
Here is an example of the first overprinted issue (January 5, 1925) from Poland...
Scott 1K2 2g dark brown (1924 "Arms of Poland")
How successful was the appeal, or did the Polish mail boxes stay up?
As Danzig was 95% ethnically German, how much demand was there for Polish issues? Apparently enough, as Poland continued to produce "Port Gdansk" overprints.
Here is a 1926 Scott 1K11A 5g yellow green (1925 'Poznan Town Hall")..
This stamp issue must be scarcer, as it has a higher value in Scott.
Finally, here is a 1929-30 series...
5g dark violet, 10g green, 15g red brown (Polish 1928-29 "Eagle Arms" issue)
Bee See's note says the Danzigers were nervous because of the recent invasion of Memel by the Lithuanians. Well here is a 1930 map of Poland, and the Danzig area (port) look to be part of it.