Very nice find. Mediaeval seal matrix it is.
Reversing the image and reading clockwise from the cross at 12 o’clock it looks to read "+SIGILLI….” Followed by some letters I can’t make out. Perhaps “TRID…”
View attachment 2014181
‘Sigilli’ is the genitive of ‘sigillum’ (seal), so commonly these matrices have inscriptions that begin with things like “SIGILLI MEI…” (my seal) or “SIGILLI NOSTRI…” (our seal); or the word sigilli is followed by a name, authority or religious dedication, which looks to be the case here.
That’s as far as I can get you without being able to read the rest of it in full.