Yes, you are correct in that the flowers are not quite the same has those shown in Pilbeam's book. For me the photograph in the book lends more toward M.chica but I believe this is the true M.gasseriana sensu Boedeker and this has be 'tentativelty ' confirmed by the Fitz-Maurice's.
I have some seedlings of M.magallanii hamatispina TL 103, San Rafael, Coahulia and these are looking very similar to above plant with the shape of the body and the spines, but they will need a few more years of growth to be sure.