Although the flowers could point to M. laui, the rest of the plant does not really fit to laui. It is too columnar for a laui and the spination does not conform to laui. But what it is? I don't know. It looks more like a white-spined form from the polyacanthae-series, perhaps backebergiana or spinosissima. But whatever it is, it is an attractive plant with its white spines and its purplish-pink flowers.
Wolter ten Hoeve.