When I first looked at the photo, I thought of M. voburnensis rather than M. karwinskiana. It looks to me as though it has at least 1 central spine, and the colouring of the plant is more typical of voburnensis that karwinskiana. There is a form with longish spines which was called voburnensis v. quetzalcoatl, but I'm not sure that any subspecies are recognised now.
There are similarities between these two species, and I wouldn't get too worried about the lack of wool on the areoles, as this can vary a lot and can be cultivation and light dependant.
I'm not a 100% sure though, so it would be good to have some other members' views.....