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What is this card, and why does it go “on the hat”?

This document is from the papers of a family whose ancestors immigrated from Germany to the United States in the late 19th century:




9 Centralbahnplatz 9

New York
1 Broadway
gegenüber Castle Garden

Aelteste bedeutendste General-Agentur für Auswanderung

Amerikanische Bank

Einzug von Erbschaften, Besorgung von Liquidationen

and then on the sides, in German and French:

Diese Karte ist vor Austritt aus dem Castle Garden auf den Hut zu stecken.

Prière de fixer cette carte sur le chapeau avant de sortir du Castle Garden.

Zwilchenbart is the name of a bank in Basel, about which the U.S. consul in Basel writes in 1900:

It has been for very many years occupied with the transportation and with the financial arrangements of travelers, tourists, and emigrants.

This fits with the card's characterization of Zwilchenbart as "[The] oldest [and] most important general agency for emigration" (my translation).

However, I don't see anything that says what this card is for. The bilingual instruction on the side only puzzles me more:

This card is [for one] to stick (pin?) on [one's] hat before leaving Castle Garden.
(my translation of the German)

Castle Garden was New York's immigrant processing center from 1855-1890. The card indicates that Zwilchenbart has a bank branch opposite Castle Garden.

My best guess is that the card is a bank membership card, but if so, I'm puzzled that it seems entirely generic and doesn't bear any information about the account holder. What is this card for, and why are immigrants instructed to attach it to their hat?