Identification of lophophora

[vim fuego‏]

just found the group - looks great for all things lophophora - hope someone can help. i have just acquired this loph. i didnt get it from a nursery or known cacti supplier so i'm not quite sure what it is.
 
i've attached a photo, the main body is about 3.5cm and it has several pups - it was sold as a caespitosa. however, looking at pictures elsewhere on the web, is looks more like a diffusa in terms of rib pattern - most of williamsii i see have 5-star look etc..
 
i read that caespitosa variants are always williamsii...can anyone give me a definitive ID?
 

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