I suspect that the tiara marked number 4 in Kasper's photos
is not the tiara worn by Frances, Lady Muir Mackenzie (a sister to Georgina, Lady Dudley), rather the jewels are similar.
Unfortunately, I cannot find another photo of any of the Ladies Muir Mackenzie wearing the jewel as a tiara -- and of course the photo of Frances, Lady Muir Mackenzie lacks definition.
The Muir Mackenzie jewel was worn by a subsequent Lady Muir Mackenzie as a necklace, but once again the quality of the photo is not brilliant.
What do others think?