As others said, a love token. Each of these is their own piece of art, made by one person and given to another. That individuality means that there is no official tracking of these things, sort of like Hobo Nickels, whoever wanted to make one could make one. That sets it apart from alterations done in official capacities like Reales being counter-stamped to be a different currency but hold the same silver weight.
The best you'll find online will likely be a gallery of examples, if such a thing exists online. There won't be any official info because each of these often bears one or both initials of a pair of random lovers from some time in history
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