"After 1906, all naturalizations were in the federal district court..."
Although this is also stated on some web sites (and presumably in some
books, too), it is not entirely correct. Beginning September 26th, 1906,
with the creation of the U.S. Immigration and Naturalization Service and
standardization of naturalization laws, federal courts were required to do
naturalizations, but other (e.g., state and county) courts were not
required to stop processing declarations of intention and petitions for
naturalization. Many of them did, to avoid the paperwork and regulatory
requirements, but the decision to stop was apparently made on a
New York, N.Y.