Others have beat me to it, but my guess was oysters, partly because I know they were sold on the street, however they were popular with all levels of society so I'm not sure. Diane's right that poor man's protein has been beans and legumes throughout history, but I can't think of a street food of the time that had beans in it, except regionally (memory serves, there were various Mexican dishes sold on the street in the South West cities and parts of California). Although the difference between street food and restaurants wasn't always clearly drawn back then, so some bean dishes might count I guess.