By the way, I now recall that "Diamond Diadem" is used often in Queen Victoria's JournaIs when it is plain that her Regal Circlet is under discussion.
Is it the Diamond Diadem I posted that you are discussing here? It has floral sprays rather than fleurs de lys.
There is a lot of information about work done on the Diamond Diadem. All elements were removable (from memory) but at least that front cross is now fixed in place.
Now checking Roberts 2012, Crown Jewels 1998, and my notes.
Edit: From the Crown Jewels 1998 - "The crosses paty are numbered I-IV and correspond to similar Roman numerals inscribed on the band. Three crosses are screwed on to the band; the fourth, in consequence of a repair, is now permanently attached." We can see the two screws used on the back cross.