About 27 years ago, Michel Lacoste, Felix Otero and Christian Brachet made a series of articles which was published in the Mammillaria Society Journal.
The website for this Society now holds the first 4 of these articles, which may hold both interesting information about their travels and the pkants they have found, plus their observations upon nomenclature.
The website is here: Mammillaria Society
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Chris43, moderator