According to the index card one John Spahn was naturalized 24 Feb 1858 in the District Court of Dubuque Co, Iowa. The other John Spahn was naturalized during Oct 1963 in the County Court of Dubuque Co, Iowa.
You could contact the courts to see if they have the records or if they have been transferred to an archives.
By the way S-150 is not a card or form number. S-150 is the soundex for the surname Spahn.
Dear Ms. Starling,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located a series titled the Administrative Files, 1850 - 1924 in the Records of District Courts of the United States (Record Group 21) that includes records for the U.S. District Court for the Northern (Dubuque) Division of the District of Iowa for 3/3/1849-7/30/1882. For access to these records, please contact the National Archives at Kansas City (RM-KC) via email at email@example.com.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RM-KC. We apologize for this inconvenience and appreciate your understanding and patience.
If you are unable to locate naturalization records , and as the previous poster suggested, please contact Dubuque County to request a search of their archived records.
We hope this is helpful. Best of luck with your family research!