Kosher rules banished the dairy offerings of German delis from the Jewish deli, A second sort of establishment, the appetizing, was created for cheese and other dairy products, but it was preserved fish that was their real specialty. Zabar's on the Upper West Side started as an appetizing; Barney Greengrass and Russ & Daughters are among the few that are still around. You went to the appetizing for black lumpfish caviar, kippered salmon, smoked sturgeon, sable, herring and, of course, the lox and nova, which often took the place of bacon on the Sunday breakfast table for New York's Jews.
The stores often also sold bagels to go with. As Arthur Schwartz points out in New York City Food , bagels, which had originated in Poland, were not the giant fluffy, bready, thick, donut-shaped bread with a soft crust widely sold as bagels today in supermarkets. They were small and dense and were almost exclusively found in Jewish neighborhoods in 1946. The crust was chewy and the interior soft and fragrant when fresh from the oven. You needed to reheat them after an hour or two but you never toasted bagels unless they were a day old. They could have poppy or sesame seeds or onions. They did not contain blueberries or raisins, Mimi Sheraton wrote a book, The Bialy Eaters, on her effort to trace the origin of a popular variation, the bialy, from the Lower East Side to Bialystock. Maria Balinsky has written an anecdotal history of the bagel, The Bagel: The Surprising History of a Modest Bread .
Schwartz says the brine-cured salmon known as lox arrived in New York when the transcontinental railroad made possible the importation of Pacific salmon in the mid-19th century and later was adopted by the Jews who were used to other cured fish in their homelands. On the other hand, a Wikipedia articles says that the Eastern European Jews brought it to the city. My guess is that lox, which is widely eaten throughout northern Europe, was consumed in the city before the mass immigration of Jews from Eastern Europe but that the immigrants already were familiar with lox made with Atlantic salmon before they got here. The name lox is derived from the Scandinavian name for salmon and is virtually the same in German, Yiddish and Old English. Nova is the milder, less salty version. According to Schwartz, the popular combination of cream cheese and lox was an invention of Kraft popularized in the '30s on Al Jolson's radio show. Oldtimers had never heard of such a thing.