The flower is radially symmetrical, therefore S. x buckleyi. If S. truncata it would be pointing to one side, looking at it from the side, usually pointing down. If S. truncata, looking at the opening, there would only be one axis of symmetry. S. x buckleyii flower is symmetrical from any axis (radially symmetrical.)
http://cactus.biology.dal.ca/p...
What about it makes you question the ID? Something besides what's in these 2 pics?