Unfortunately I wasn’t able to find out about St Christopher specifically, I did find this about medals in general. Hope this gives you some answers.
*While it is probably that the traditions described above continued into the Middle Ages and never entirely died out, little evidence has been found to support the use of medals during early medieval times. Although p(name removed by moderator)ointing the date at which point one first hung a medal around the neck with a religious intent is, for obvious reasons, quite difficult, no trace of such sacred objects survives which remarkable either for artistic skill or the value of the metal in this period.
But a little later, in the course of the twelfth century, a very general practice grew up at well-known places of pilgrimage, of casting tokens in lead, and sometimes probably in other metals, which served the pilgrim as a souvenir and stimulus to devotion and at the same time attested the fact that he had duly reached his destination. These signacula (enseignes) known in English as “pilgrims’ signs” often took a metallic form and were carried in a conspicuous way upon the hat or breast. Giraldus Cambrensis referring to a journey he made to Canterbury around the year 1180, ten years after the martyrdom of St. Thomas, describes himself and his companions returning to London, “cum signaculis Beati Thormae a collo suspensis” (“with the tokens of St. Thomas hanging round their neck”).[3] Again, the author of Piers the Plowman writes of his imaginary pilgrim:
An hundred of ampulles on his hat seten,
Signes of syse and shelles of Galice;
And many a crouche on his cloke, and keyes of Rome,
And the vernicle bifore, for men shulde knowe
And see by his signes whom he sought hadde
The “ampulles” probably represented Canterbury, but may have been tokens of the Holy Tear of Vendome.[4] The “shelles of Galice,” that is, the scallop-shells of St. James of Compostella; the crouche, or cross, of the Holy Land; the keys of St. Peter; the “vernicle,” or figure of the Saint Veronica, and so on, are all very familiar types, represented in most collections of such objects. The privilege of casting and selling these pilgrim’s signs was a very valuable one and became a regular source of income at most places of religious resort. From about the twelfth century, the casting of these devotional objects continued until the close of the Middle Ages and even later, but in the sixteenth or seventeenth century, they began to be replaced by medals, properly so called, in bronze or in silver, often with much greater pretensions to artistic execution.*
Here is the link:
newworldencyclopedia.org/entry/Devotional_medal
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