Doubts about identification

I think that this mammillarias is a "Karwinskiana ssp nejapensis".
The plant has four heads.
I hope in your help, thank all.
Here photoes:

http://www.flickr.com/photos/53461448@N03/5192158626/
http://www.flickr.com/photos/53461448@N03/5191562887/in/photostream/
http://www.flickr.com/photos/53461448@N03/5191562721/in/photostream/

Yes, I would agree. The lack of radial spines, short upper spine and wool and bristles all point to that as the right identification.

Thank you!
I'm very happy: slowly i'm getting into mammilaria's world but more studies still has to be done.
Searching the match is challenging and i feel it's a good training to undestand these beautiful plants.