I think this is M. haageana ssp san-angelensis, which has been called M. elegans var dealbata. The various M. elegans subspecies and varieties are now variously assigned to different M. haageana subspecies. Most M. haageana have darker central spines and less striped flowers, which is why I chose san-angelensis even though you don't see many about. It may also be a M. haageana/elegans of somewhat mixed parentage as many of them are in cultivation :)

looks rather like my unknown mamm.

I'm not familiar with ssp san angelensis, it's a nice plant I like the dense wool.
I have bought several similar plants under the somewhat dubious name of "Mamm. fuauxiana elegans", (fuauxiana is now considered synonymous with albilanata). The two that have flowered so far have the typical small red flowers of albilanata.

Cheers, CP

Thanks for the help. After checking around, I believe that M. haageana ssp san-angelensis is spot on.