So far I haven't found an image proving what brooch this was; however, I believe that it matches this brooch
As usual I checked to see if there was any published material on the origin of this brooch.
The only site which has an item on this brooch is the Instagram site, British Royal Jewels which posted an item on 28 February 2021. The site owner didn't identify the origin of the brooch noting only that Princess Margaret had a number of floral brooches and that this one had been inherited by Lady Sarah Chatto. See https://www.instagram.com/p/CLzYV1DFODT/
Princess Margaret was photographed wearing this brooch from March 1956
In the image above Princess Margaret can be seen wearing her diamond and sapphire earrings.
The Royal Watcher published an item on them only yesterday. See https://royalwatcherblog.com/2021/05/01/princess-margarets-sapphire-earrings/
This brooch remained one of Princess Margaret's favourites over the years.
In June 2002 Lady Sarah Chatto wore the brooch to the ceremony at St Paul's Cathedral for QEII's Golden Jubilee.