Question concerning the "U.S. Subject Index to Correspondence and Case Files of the Immigration and Naturalization Service, 1903-1959"

I'm researching a 1912 federal prisoner named Chris Lewis. I located him on the US Subject Index to Correspondence and Case Files of the Immigration and Naturalization Service, 1903-1959 [image #3247] He's listed on the bottom of the card (see attached image) under the name Stassonopulos, Chris and #612,341.

My question is this -- where do I go to learn more about this individuals case?  I can see from the top of the card that this is #3 and it says Immigration Officials (See also Inspector in Charge of Commissioner of Immgm). But what does the #612,341 refer to and where do I look for this information?

The Roll Content Description lists GREECE LEGISLATION -- IMCOME TAX • ROLL #16. 

I also know (about him) that he was in Leavenworth Penitentiary from September 1912 to December 27, 1916 when he was discharged. He was in prison for 'white slave traffic' aka THE MANN ACT. His prison file (at McNeil Island) states that he had a letter to/from his father, George Lewis who lived at 299 4th St. Milwaukee, WI. He was attested in LaGrande, OR (1st offense) • plead not guilty • sentenced to 7 years (McNeil Island) and transferred to Leavenworth Penitentiary in September 1912.  I also believe that he was born in 1885 in Greece. I've not been able to locate him in any census records; passenger lists; WWI selective service records; or city directories. Basically I know nothing about him prior to August 16, 1911 or after December 27, 1916.