Not seeing any reason
Not seeing any reason to deny the Berlin World Pro Championships 1932 & 1933 and the Bristol Cup (1920's and early 1930')- Pro Major status. They were regarded as amongst the Major Pro tournaments at the time. Hence, that is good enough for me. My only question is about the Bristol Cup in the early 1930's - was it still regarded as a Major then with Berlin being around in 1932 and the French Pro being around in 1930 and 1931?
If you add them to the list of Wembley, French Pro, US Pro, Tournament of Champions and Wimbledon Pro - then it still ensures that there aren't more than 4 Pro Majors in any one calendar year.