thank you for yours very interesting informations.
I know only this about your mammi (see here)
"(..)This is one of the Mammillaria commonly called "Owl Eye Cactus", known for dichotomous branching (forking or dividing into two parts). Although dichotomous branching is not a common occurrence in cacti in general, it happens for some reason in this particular subspecies. What is interesting about this cactus is that it began as a single head, and it has now divided twice, forming what will be four separate branches. When the division process started, it was obvious that four heads would appear, but I don’t think the one head divided quadruply. Most probably, one head became two, and then those two immediately divided. (..)"
About my M. sempervivi post: the plant is or was at Jardin Botanique Collection de Porrentruy.