The following information was found on one of the Minnesota Mailing Lists. It is reprinted here with the author's permission.
In my search for immigrant ancestors, I have come across an interesting document on many of them. I have not seen these mentioned for other states, but in Minnesota, any immigrants who were not naturalized by Feb., 1918, had to complete an Alien Registration and Declaration of Holdings form.
This form is interesting in that it had 35 questions to be completed by the registrant, including the basic name, address, birth date and place. Also recorded were the place and ship of arrival, occupation and whether or not the person wrote or spoke English. One question required names and ages of any living children, another whether there were any relatives with any foreign army warring with the US and its' allies, and yet another whether the individual filed for the WWI draft, whether the first papers were filed or not and if not, why. The individual also was requested to list any property owned by them outside of the state, and then list anything of monetary value they presently owned.
I have found this form to be helpful in gaining some insight into some of my ancestors. I do not know whether these were filed in other states, although I have been told that NARA has Alien Registration cards--they do not contain as much personal info as these registration forms do from what I read. The forms were processed under the Commission of Public Safety and
were to be forwarded to the State Auditor, at least in Minnesota. I found mine at the MN Historical Society, indexed by county. I believe I also saw them at the Ironworld Research Center in Chisholm, but this may be for only the northern part of the state.
If you found your ancestors on the 1920 census, and know that they arrived before 1918 and they stated on the census that either they're still alien (AL) or filed first papers (PA), then there is a good chance that one of these registrations would be on file.
Hope this helps someone.
Anna in MN