Hi Ralf,
The plant you show is definitely one of the Polyacanthae series, of which the hooked spines species include nunezii, duoformis and rekoi.
I find it quite difficult to be certain about an identification as the hooked spine is quite a variable feature of this Series. It looks quite like a couple of plants that I have which have been named as nunezii, basically because of the lower number of radial spines, compared with rekoi and duoformis. There's not too much to choose about flower colour or shape between them.
However, we know that number of spines, especially when comparing plants in cultivation with those in habitat (from which species were properly described), is also not the most robust of characters.
Maybe some others here might be able to do better!
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Chris43, moderator