That is the frame of the crown of Queen Adelaide which was dismantled almost immediately after the coronation in 1830, after which Queen Adelaide's Circlet was made.
I have only ever seen one depiction of Queen Adelaide's circlet, being worn by Queen Victoria in a tiny miniature, however this has since been removed from the Royal Collection site.
As far as I am aware the empty frame of this circlet no longer exists, I have never seen any images of it or read of any mention to it. After the circlet was dismantled in 1853, it seems to have been disposed of.