I have this in Lau's list as sp. heyderi Group from Monterrey, Nuevo Leon, 1000m. From the photo I count 6 radial spines, 1 central, white flowers. M. melanocentra has pink flowers, 7-9 radial spines, 1 central. M. linaresensis has 4-6 radials but 2 central spines, white flowers. M. heyderi has 15-22 radial spines, 1 central, white flowers. M. gummifera and M. meiacantha are closer in term sof radial spines, but have striped flowers.
M. melanocentra and M. heyderi are in the same group in Hunt's classification. Luthy has melanocentra in hi magnimamma group, with heyderi in its own group. Reppenhagen list sthem as numbers 237-9 for heyderi and if forms, and 249 for melanocentra.
So even the experts are not unanimous in just how closely related these plants are.
I think it is more likely to be just a form of heyderi.