Hi Mercia, and Thord,
The flowers look right for M. matudae, and seem that the plant should be from the Polyacanthae series, but I'm not convinced that the spines fit that actual species. I know that matudae is quite variable, so I can't be sure.
I count 4 dark central spines arranged in a cross layout, with the lowest of these just about the longest, and about 20-25 radial spines, the lower thinner, the upper stronger.
My thoughts are tending towards a form of M. nunezii rather than M. matudae, but I am just sharing thoughts rather than giving a firm opinion.
Do you have a picture of the plant from the side, which perhaps will show the spines better