More or less, not a complete answer, but a partial answer to this question can be seen from around 1577.
Here in the Florentine Codex,
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you can see that the text references to the images with simple numbers (11...12....etc) at the end of the text.
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You can see that references in the text to other pages resembles the modern way of making a reference to "folio 48".
background info:
an encyclopedic work about the people and culture of central Mexico compiled by Fray Bernardino de Sahagún (1499–1590), a Franciscan missionary who arrived in Mexico in 1529, eight years after completion of the Spanish conquest by Hernan Cortés. Commonly referred to as the
Florentine Codex, the manuscript consists of 12 books devoted to different topics created around 1577
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for example the "...los arboles, y trastorna las paredes: y levanta grandes olas, e nel agua, las canoas qua topa e nel agua, echa las afondo, olas levanta enalta.."
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