It is very large, 18.5cm in diameter. The medallions around the outer rim of the clasp are in fact little philatories, transparent reliquaries which would once have contained relics. It's possible to see tiny labels inside, of the sort that generally accompany relics giving a description, but the relics themselves are absent.
I couldn't help thinking what a magnificent addition this would make to some of the Copes we are asked to repair. I'll definitely be having a look on Etsy to see if I can find anything similar!