I found a plausible explanation here: http://www.network54.com/Forum/27140/message/1296951241/More+Numerology-+concert+pitch+and+the+equally+tempered+scale.+Wa

I found a plausible explanation here: http://www.network54.com/Forum/27140/message/1296951241/More+Numerology-+concert+pitch+and+the+equally+tempered+scale.+Warning-+long+post http://www.network54.com/Forum/27140/message/1296951241/More.... tl;dr: The BBC reference pitch was generated by dividing and multiplying a 1MHz crystal oscillator. A440 was achievable by this means; A439 was not.