I didn't remember that they came from the Mammillaria Society, but I have just read the Notes of the Seed Distribution, which sadly isn't much help as it says right at the end, "......sp. Matehuala MA 234.2 about which we know nothing".
Checking Cactus Heaven website, I see that MA234.2 is not, as I had assumed, a field collection number, but simply the catalogue reference number for this seed.
Thanks, Ento, for that information. If you have a photo of an older plant it would be interesting to compare.