I think he is referring to just the letters "L" next to every entry of born in Russia, speaks Yiddish that appear on the manifest. It would seem that they part of some post-enumeration statistical survey. The only combination that receives the "L" are born in Russia, speaks Yiddish. For those born in Russia, speaks Polish, there is a letter "O." The "O" also appears for born in Austria, speaks Polish.
I suspect that this analysis was done to provide data in connection with the drafting of the Immigration Act of 1924. The Act limited the annual number of immigrants who could be admitted from any country to 2% of the number of people from that country who were already living in the United States in 1890. The law was aimed at further restricting the Southern and Eastern Europeans, among them Jews who had migrated in large numbers since the 1890s to escape persecution in Poland and Russia.
The law sharply curtailed immigration from those countries that were the homelands of the vast majority of the Jews in America, almost 75% of whom came from Russia alone. Because Eastern European immigration only became substantial in the final decades of the 19th century, the law's use of the population of the United States in 1890 as the basis for calculating quotas effectively made mass migration from Eastern Europe, the home of the vast majority of the world's Jews, impossible.