I have not yet found a match to this 20 year old. Can anyone help? The M. Backebergiana seems very close but I cannot recall ever seeing a flower. Other ones that seemed a bit close in some pics:
Mammillaria scrippsiana
Mammillaria cirrhifera
Mammillaria karwinskiana
Mammillaria petersonii
Mammillaria bonavitii
Mammillaria zeilmanniana