Looking around at old Vogue issues, I discoverd why. The original dressed was designed by John Bates a.k.a. Jean Varon for his Avengers collection. Take a look at Diana Riggs sporting it along with the coat in an Avengers episode:
I also found Jean Shrimpton wearing the coat over the dress in a British Vogue 1965 issue, photographed by Bailey (small photo, bottom right).
(Vogue scan courtesy of Youthquakes - Avengers photos courtesy of Vintage Chic)