I think you are right, Arthur, with M. backebergiana. The central spines vary a lot in this species, according to the description, from 1 to 3, light brown to dark brown. I have a plant in my collection that has no central spine, or rather the one that might be a central is pale like the other radial spines, and pointing upwards almost in the same plane as the radials.
Without doubt, they are plants from the Polyacanthae series, and the Spinosissima group, which includes backebergiana, matudae, spinosissima itself and its variants, and meyranii. I favour them all being backebergiana.