Moishe,  this USCIS webpage has links to the various types of files available from the Genealogy Program: https://www.uscis.gov/history-and-genealogy/genealogy/historical-records-series-available-from-the-genealogy-program .  If you follow the link to each file type, you will be presented with a description of the contents, as well as an aid to finding the file number, if the number is available outside of USCIS.

For Registry Files (https://www.uscis.gov/history-and-genealogy/genealogy/historical-record-series/registry-files-march-2-1929-march-31-1944 ), there are some cases where the number can be found outside of USCIS.

For Visa Files (https://www.uscis.gov/history-and-genealogy/genealogy/historical-record-series/visa-files-july-1-1924-march-31-1944 ), the number isn't available outside USCIS & must be obtained from an Index Request.


Regards,
David Oseas

Researching:
HYMAN/HEYMAN/HEIMOWITS/CHAJMOVITS: Zemplen-Dobra, Hungary > New York;   KLEIN: Satoraljaujhely (Ujhely), Hungary > New York > Los Angeles
KRONOWITH: Hungary > New York;   OSEAS/OSIAS/OSIASI/OZIAS: Iasi, Romania > Chicago > Milwaukee > Los Angeles
SCHECHTER/SHEKTER: Kishinev, Bessarabia > New York ;   SHERMAN: Iasi, Romania > New York > Los Angeles
STRUL:  Iasi, Romania > Haifa, Israel;   WICHMAN: Syczkowo (Bobruisk), Belarus > Milwaukee > Los Angeles