The ORB2.3 looks good to me, and Monterrey, NL is the type location, I believe.
But I don't think the MG949 photo is a proper M. winterae. It is supposed to have only 4 spines and yellow flowers. Even Mesa Garden catalogue for MG949 says yellow flowers. I'm not sure exactly what it is, but it does have a resemble, for me at least, to M. melanocentra. This also comes from Nuevo Leon around Monterrey, but I'd have been surprised if Steven Brack confused the two, especially with the flowr colour difference. So I suspect a mix up at the seedling stage.
I found the photos of winteriae on Fotki, but I dont think that is a winterae either. Not just because of the flower colour, but also because it is a clustering plant, and winterae is solitary. It also has a lot of axillary woll and winterae doesn't. My feeling is that is is closer to on eof the many forms of magnimamma or compressa, but am not sure. M. winterae ssp. aramberri, or M. crassimammillis does cluster, but it has yellow flowers as well.
What do othes think?