I noticed that PaulP, one of the commentators on The Court Jeweller site, has identified an occasion in 1957 when QEQM looks to be wearing the two large sapphire brooches at once. One is as a brooch and the other is as a clasp to her pearls. For PaulP's comment scroll down at https://www.thecourtjeweller.com/2021/11/the-night-princess-diana-wowed-at-the-white-house.html#more-35686

The video he refers to is on the Royal Family channel see



Below are some stills. I cannot make up my mind, but this is not one of the clasps I have seen previously.

What do others think?