This is speculative but if you are at a dead end, who knows. I incidentally recorded several ARKO, naturalization filings from Denver
Vol. # Page # Cert. # Name (last, first) Country of Birth Renounce Allegiance to: Year of Birth Year of Arrival Year of Filing intent
21 478 9871 ARKO, Frank Yugoslavia 1892 1908
17 452 7845 ARKO, John Jugoslavia, 1873 1905
6 90 1979 ARKO, John Austria 1874 1903
SOURCE: The Foothills Genealogical Society, Index to Naturalization Declaration of Intentions File, Denver District Federal Court, 17 April 1877 to 3 December 1929. Rootsweb. 29 Apr. 2007. I have a link (URL) but not sure it can be included in this post?