Color looks right for silver. Is it heavy? if it were solid silver it would seem a little heavy for it's size. It being from Mexico, it could be a low grade silver and that may be why it is not marked. Only way to know for sure is to acid test or to scratch it inside somewhere to see if it is plated. If you have a good eyeloup you can sometimes see where plating is bubbling or has already came off around a hinge or clasp.
Either way, Nice find..Grats