Obligatory disclaimer: not a historian, or an expert on any of these things really, so don’t quote me. But:

There’re a few things a bit off in that site’s description:

one side with clasped right hands with the inscription “LA FERME ME CONTANTE” (the constant faith), and on the other side a Maltese cross with “S MARIA IESV” (Saint Mary Jesus). The initial “F” is engraved inside the lid and box (maybe the initial of the bride or shared surname?).

First, it doesn’t say ‘S MARIA IESV’, it says ‘MARIA IESVS’ (ie 'MARÍA JESÚS), you can see the stars separating the two names.

Second, ‘LA FERME ME CONTANTE’ doesn’t translate in Spanish to ‘the constant faith’, in fact it doesn’t translate to anything, that’s not Spanish. I think it’s French (my French’s not great, much less my 17th century French), and I would say it means ‘Steadfastness (not quite sure, it could mean literally ‘the farm’) contents me’.

Third, we don’t have shared surnames in Spain, never have, so the ‘F’ most certainly isn’t that. Since it’s hidden inside the lid and box it could be the mark of the goldsmith that made it, I don’t really know.