While I have not been able to find photos of this hat in action, it follows a similar formula of millinery design that Fox and the Queen’s other milliners followed in the late 1960s and 1970s using a turban shape and  trailing embellishment down the back. It’s a time-rooted style that doesn’t translate well to present day but became a signature millinery look for Her Majesty during this era. If forced to choose a favourite here, my vote is with the Fox design displayed above. I think the combination of gossamer light feathers and smooth silk chiffon makes a more elegant and much less clunky design than the fussier ones below.