Mammillaria Lau0621, marksiana ?

You surely are right that the first picture is not marksiana. But just in case, do you remeber what colour flowers it had? bocensis is supposed to have pinkish flowers, although in its neoschwarziana incarnation that pales to almost white.

Your second picture is a bit more strongly spined than my only plant (without field data) and is a lot browner in spine colour, mine being quite a yellow plant indeed, and is almost identical to the picture in Pilbeam of Lau 621.

Plein and Rogozinski say in their articles on this area that they will come to marksiana in more detail, but I haven't seen anything in recent issues. The suggestion is that there are red flowered forms of marksiana, and that it as a species might be in question - again! So your question is, I think, very timely - but I don't have any answers...

The flower colour on Bob's plant is a pale lemon colour with a pink mid- stripe on the outside, I remembered to take a photogragh at the time. In the article in AfM 31 Rog states that the M.bocensis from around Bacubirito have yellow flowers so that fits with this.

Sounds entirely possible to me Mark.

I have been in touch with a number of people inculding Hunt, Plein and Bob Stanley on the subject above and would like to give an update.
I have a copy of Lau original data card and it gives the following details. Collected L621 on 30-4-73, Rancho del Pardre, Bacubirito, Sinaloa. Altitude 800m, Latitude 25/35n, Longitude 107/40w. Descirption of habitat; In deep valleys on almost vertical cliffs, mostly facing south. The mountains rise to the Sierra Madre Occidental.
The way LAU operated was to revisit the same plants, in the same habitats, every year or every few year's and with his boy's collect seed, his wife Annie would clean it, then they would sell it on to raise funds for his home for boys. In a Newsletter from June 1986 Lau writes '' During these most difficult summer months when correspondence is rare to be received, we would be especially grateful to hear from you, as in that time we shall work with 12 boys in Chihuahua, Sonora and Sinaloa, collecting seeds and suffering in the intense heat'' It appears that the seed from which this plant was grown came from this collecting trip. This collecting trip would seem to copy his original trip of 1973. The seed was received in 1987 and held over until it was sown in 1988.
I E-mailed the above photo to Plein who compared it to plants of M.bocensis from Bacubirito in his collection. It was far from conclusive, so I intend to send the plant to Plein for a better comparison. But Plein did say that the area where Lau collected Lau621 was flooded to create a reservoir. He speculates that both plants grew together and that Lau's boys collected seed (1986) of both M.marksiana and M.bocensis at the M.marksiana site and mixed up the seed and this led to it being sold has L621.
I will finish this by saying that hunt believes the M.bocensis from Bacubirito are better refered to as M.sonorensis which is the older name (1940)