To give you a couple of examples of things you should know if you collect used Danzig:
This is a '24-hour' postmark (note the time: 17-18). A genuine used would have 'V' for Vormittag (before midday) or 'N' for Nachmittag (after midday).
This 'postmark' was created from an old French Handbook:

Postal History is - as always - dangerous.
Here you can see the date/year it was used - 1920.
The phrase "Freie Stadt Danzig" was not used before 1921.

(Originally published by BPDh)