Field called this tiara a property of MF. But I think it's the Russian Palmette (now with the Glochesters) and was made for QM in 1914. But may one of the sapphires (in the middle part of tiara) or the brooch is from MF

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The brooch is the one with the drop pearl from MF.
The tiara again you are right in saying belonged to Queen Mary before WW1 (it was comissioned by her) the sapphire centre only was bequeathed to Princess Margaret who wore it as a brooch...it was not sold by her children..it was not a snake design.
Fieldes did say it belonged to MF but as we know...she did not save any tiaras when she fled Russia.