According to Pilbeam, p155, ssp. nejapensis has only radial spines. However, several of my plants withthat name appear to have quite long white spines, but it is difficult to say whether they are radial or central. Your plant looks a little closer to the picture of ssp. collinsii shown on P154 that that on P155, but my collinsii appear a whole load darker than your does.
ssp. collinsii has vacillated between karwinskiana and voburnensis, I believe the Germans favouring voburnensis because that is where Reppenhagen put it, whereas it is Hunt who has put it into karwinskiana.
As it ages, it should offset not divide dichotomously, and that will (hopefully) be the final word.
I wonder how far south is south of Oaxaca City? A certain distance would put it into nejapensis territory, but further still, like near the Pacific Coast, it is in colllinsii land. There could then be some intermediate forms, and maybe this is one of them!
I would keep it as labelled for the moment, and see how it develops.