I am told that the guys at the UK's National Cycle Museum have no knowledge of a curved seat stay Carlton frame.
The seller's father was apparently a Carlton bicycle collector and had several models. As far as this bicycle is concerned, he suggests that the frame might have been made by Carlton for his father, specifically for him.
He is not aware of any connection that his father might have had with Carlton.
This information seems to answer the question, for the time being at least.