It is Mammillaria multiseta Rep 831. This species was named a long time ago, ad without clear typification. Some collectors believe they have rediscovered it and it is a good species. Others believe it is just a form of M. karwinskiana. It is close to this species certainly, but I believe that it offsets rather than divides dichotomously. I suspect that is not enough to justify a species name of it sown.