Re: Where to find Illinois Northern District Naturalization Records? #records
The card indicates the naturalization took place in the/a "municipal court" in Milwaukee (presumably Wisconsin) on March 16, 1906. Looks like it is petition # 9024 in volume 15, page 31 of that court's naturalization records.
That is not a Federal court, so will not be at the National Archives. You'll need to track down the records of that court. The FamilySearch wiki says this:
The Milwaukee circuit and municipal court records of 1836 to 1941 are at:
Milwaukee County Historical Society
910 North World 3rd Street
Milwaukee, WI 53203-1591
At least that is a place to start. Good luck,