Interesting question, Rumncoke, and Wikipedia has the answer. It was called the Bellamy salute, named after Francis Bellamy, the Christian socialist minister and author who wrote the Pledge of Allegiance. The pledge and salute were instituted in 1892.

Mussolini's Fascists adopted what the called the "Roman Salute" in the 1920s, and the German Nazis quickly copied it. The U.S. instituted the hand-over-heart salute when Congress amended the Flag Code on December 22, 1942.

So, maybe the Queen Mum was doing an American salute?

-Joe-