The naturalization act of 1906 became effective 9/27/1906.

Generally, as of the above date, naturalizations occurred in:

(1) United States District Court [USDC]
(2) United States Circuit Court [USCC]
(3) A court of competent jurisdiction: e.g, a court at the county level, such as a superior court [Massachusetts], or a court of common pleas [Pennsylvania].

Prior to the above act, naturalizations, also, occurred in other courts, such as, Police Courts, District Courts, Municipal courts, etc. [generally courts at a town/city level].

Note: familysearch.org has New England Petitions in district/municipal/police courts prior to the naturalization act of 1906. Search in the catalog. Input the city/town. In Mass., they have a direct link to the images of the petitions. Find the date of the naturalization date from the naturalization index and then search to find. For some states, they have LDS film numbers.