Officially. It shouldn't be called five or seven. The formal name is factory 57, and factory 57 is an omitted name. It is a place name in Qianjiang City, Hubei Province. Braised prawns is the most famous dish in the night market of May 7th Factory. Now it has expanded to major provinces and cities. At least I have seen May 7 braised prawns in Yunnan. Very famous.