It is in a 5.5 cm pot.
It has 8 radial spines and 1 central one. Lower radial spines are longer.
I thought it was a Mammillaria, but someone suggested it was a Coryphantha. Yet I could not find any photo of Mammillaria or Coryphantha like this plant.
I'd like to know its exact name (binary name). Please, can you help me?