Crown is the proof mark- in Europe, there were government proof houses. After being built, ALL guns went to the proof house for testing with high powered loads. If gun fired without damage, it have been "proven", and got the stamp.
Danzig (now called Gdansk) is the city where rifle was made. It has changed nationalities TONS of times (was even it's own fee state for a while) but 18711920 was part of Germany.
1891 is the year made. Being made prior to 1899, under Federal law, it is an antique, and is not subject to the 1968 Gun Control Act.