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I have conducted a search of the National Archives Catalog and located one series titled Registers of Vessels Departing from New York, New York, 8/11/1917 - 12/31/1971 in the Records of the Immigration and Naturalization Service (Record Group 85) that may contain information about the departures from New York in 1921 you are researching. The records are not yet digitized and available in the Catalog. For access to this series, please contact the National Archives at Washington - Textual Reference (RDT1) via email at firstname.lastname@example.org.
I might suggest that you contact the National Archives of the European country where the persons may have arrived; and inquire about passenger arrival lists for ports of entry.
If the Libau you reference is Liepāja, Latvia, the website for their national archives is here:
I hope this is helpful. Best of luck with your research!
Thank you very much!
I wrote to them.
This maybe of some help
Latvia, Riga Internal Passport Holders Index, 1918-1940
Note: At the present time this collection is FREE to access.
Not quite what I need but thank you very much!