Dear Mr. Trybulec,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog but were unable to locate cargo manifests for 1956 in the Maritime records.
In 1954, the SS Nassau changed to Liberian Registry. Therefore, that company may have the manifest you seek. Another possibility is that there is a listing with the Port Authority of New York & New Jersey.
We hope this is helpful. Best of luck with your research!