Dear Mr. Stuebe,
Thank you for posting your request on History Hub!
We contacted the USCIS for assistance. Generally, the number in the "Travel Document No." column would refer to the document each traveler provided to show their eligibility to enter the U.S. In most cases that was a visa or return permit. In this case, it appears to be an Immigrant Visa (that’s the “I”). A visa number was issued by the Department of State at a US Consular Office abroad (each office used its own sequential series), but the Immigration and Naturalization Service (INS) also issued its own, separate visa number to the same documents. Passenger lists from this time period often list the INS visa number, rather than the Department of State number. It is most likely that the number on the passenger list is Helmut Stuebe’s INS Immigration Visa number. Unfortunately, the number format shown on this form is different from most INS visa numbers from that time. They suggest that you request a copy of his Alien File (A-File) and examine the visa itself to see if that is indeed the number. Please submit a Freedom of Information Act (FOIA) request to the US Citizenship and Immigration Service (USCIS) to obtain Helmut's A-File. Here is a link to the USCIS FOIA form: https://www.uscis.gov/g-639.
We hope this information is helpful. Best of luck with your family research!
[Information provided by Elizabeth Burnes, Subject Matter Expert]