That is why I am confused! If Glorie/Cecilia's real name was Montgomery, and this child wasnt hers, then his birth name is unlikely to be Montgomery - too much of a coincidence. And if she gave him the name Pierre de Monti then that wont be his birth name either. Very unlikely, as she gave the name De Monti to another child too. So if neither are his birth name, why did she use a different name for him in the 1930's than for all the rest of his life? Maybe we can understand her dropping the De Monti and using her real name of Montgomery, and using it for the child too. De Monti sounds Italian and Italians were not popular during the war. But why David? Why not Peter, or stick with Pierre which sounds French and shouldnt have caused any problems.
In the 1939 register what surname is the other child using (the one born 1930 that you mention, who was also brought up as a De Monti)