What a fascinating article that was to read! Thanks Janet! Another interesting book.
The tiara in question was the Halo tiara.
Shawcross in his biography on the Queen Mother quoted a letter from QEQM (to Queen Mary ??) in which she said she would give QEII a small diadem from QEQM's collection.
The proof positive comes from Hugh Roberts
The Queen's diamonds p. 222. Roberts says specifically that QEQM gave it to QEII on 21 April 1944 for QEII's 18th birthday.
Unfortunately I have never seen a photo of QEII wearing the tiara.
QEQM wearing the jewel
I wonder if we can identify the rest of the jewels given to QEII for her 18th birthday??