Now you know you shouldn't trust the labeling from Mesquite Valley Growers. They're more like Home Depot, in that they get their cacti from a third party ("Cactus Dan", I believe). They certainly don't grow cacti from seed, unlike B&B Cactus Farm, located up the street.
Duly noted, Tumacoc. I'm not sure I really trust labeling from anywhere, including B&B, who often have plants without any labels at all. I even got a plant from (gasp) Mesa Garden recently whose identification (i.e., subspecies) was called into question by a very knowledgeable member of the CactiGuide forum.
Not to demean MG, however. If anyone knows what plants he is selling it's Steve Brack.
I'm still pretty new at this and while I usually check a plant's ID against the various reference books I have, I can't always make distinctions from photos in a book. Not to mention there seem to be a gazillion species of Mammillaria, many of which look the same to a beginner. That's when I like to post it here and almost always get a definitive answer, or at least a highly educated guess.
I think there is a difference between a collector and a grower. Of course, in different personalities, there's an overlap, and usually some of each going on. But I have enjoyed the passion more as I have become more of a grower and less of a collector. If that makes any sense.