Possible Identification of "Mom's Plant"

In a February, I made post about my uncanny ability to zero in on, and purchase Mammillarias, despite preferring cactus with showier blooms. In that post, I wrote about a cactus that I could never seem to identify, a gift from my mother. Here is what I wrote then:

"About ten years ago, my mother took a trip to Las Vegas. As a side excursion, she and her friends also visited the Grand Canyon. At a gift shop nearby, she spotted a tiny plant in a tiny pot with "Grand Canyon" handpainted on the side and decided to pick it up for her plant-loving daughter. She flew home to New York with it in her purse. (Can you imagine what airport security would say if you tried to put a cactus in your carry-on these days?) She told me "I thought you'd like this to go with your other cactuses." The thing is, I didn't have any other cactuses, and the only succulents I owned were some Sansevierias. My apartment had obstructed west windows, and wasn't sunny enough for succulents. But I was determined to keep my gift alive and well. I balanced the little pot directly on the top of a window sash, right near the lock, so it would get the most sunlight possible. Three years later, when I moved to Illinois, it was still alive and moved with me. In its first summer outdoors, it bloomed. It had kind of nondescript, straw-colored flowers, but I loved them anyway. It is in a bigger pot now, but it still spends winters on top of a window sash, for optimum light, and it blooms every year....

I've never been able to identify it, despite asking in a couple of different forums and poring over picture after picture. It is small, solitary and plain. But I will always treasure it. My mom died suddenly last year, so it is now something special to remember her by. It gave me great pleasure to be able to show it to her in bloom in 2005 when she visited me here in Illinois. And I'll always smile when I think about her with a cactus in her purse."

Well, I now think I know what it is: Mammillaria formosa subsp. microthele.

Mom's plant, just beginning to bloom, this week. It will eventually have a ring of these small, pinkish flowers. (This photo is especially large when opened in a new window.)

Its solitary growth habit thus far is what was throwing me off, I think. Most pictures I've seen show it forming a clump of several heads.

But, ID or no ID, I will always treasure this plant.