The belt, made of red flannel and wrapped around the belly of the victim, was referenced in one of Elizabeth Peters’ Amelia Peabody mysteries. In her early days Ms Peabody believed n many of the common myths of the time, although she was willing to learn and modernize. In one of the earlier books set around the years 1890 to 1900 she inflicted one of these belts on her young son.
I am not so positive about the belt but similar beliefs, including putting a heavy flannel insert in the yoke of pyjamas, were referenced by Dorothy Sayers in the Lord Peter Wimsey series.
Red flannel was a cure-all a century-plus ago. Red flannel was wrapped around the neck for a sore throat, and dressing the patient–and the room, in some cases–in red flannel treated smallpox and scarlet fever. Basic sympathetic magic–red is the color of fire, and as like is attracted to like, the fever is drawn to the red cloth and can be discarded. Nobody has concluded definitively if yellow cloth cures or causes jaundice, though. I’ve heard it both ways.