In 1890, Adolph Espe arrived at New York aboard the S.S. Slavonia from Hamburg. One column on the incoming passenger list (but not on the departing list out of Hamburg) is headed Death and Cause of Death. In that column, for Adolph, someone has written "6-4." Does this mean that he died on June 4th? Or something else? Six other people on the list have numbers in that column but the ink and the handwriting for those numbers doesn't match most entries on the page. I'm attaching an image.
StuX1:
StuX1:
As you suspect, that "6-4" is a code added many years after the fact and it has no connection to the intended purpose of the death-info column. To quote one authority (actually the authority), passenger lists "are frequently found to have a variety of markings, codes, and annotations squeezed into the margins and small blank spaces" throughout many of the columns. The best explanation of these codes and what they represent is the following article by the Chief of the Historical Research Branch, US Citizenship and Immigration Services:
Marian L. Smith et al., "Manifest Markings: A Guide to Interpreting Passenger List Annotations," JewishGen (http://www.jewishgen.org/infofiles/manifests/).