Here's the plant, first time in flower, and not one that I have looked carefully at before:
It was labelled Mammillaria mystax AvZ225 from Ocotlan de Morelos, Oaxaca.
It clearly isn't M. mystax, spines are wrong, my thoughts go to perhaps karwinskiana, but which form? It will be interesting to see if it dichotomises or offsets in time.
Whatever it is, it has really pretty flowers, and I haven't seen anything similar in the karwinskiana complex.
I'd welcome anyone's comments.