Is “One of the Eight Great Hammers, wielding purple-gold hammers” true?

As an important member of the Eight Great Hammers group in "The Complete Story of Yue Fei," Yan Chengfang's image is deeply ingrained in people's minds. The novel's original text writes: "旁边走过一员小将，乃是严奇之子，名唤严成方，年方十四，使一对八棱紫金锤，猛勇非常" ("The Complete Story of Yue Fei, Chapter 51"). However, there is no record of this person in the official history "History of Song" nor in various unofficial histories and notes from the Southern Song dynasty. Like Di Lei and He Yuanqing, he is a fictional character created purely by the novel to enrich the martial lineup of the Yue family army and enhance the legendary color of weapon combat on the battlefield.