This cactus was flowering in early June.
Also known as Parodia ottonis ssp horstii, it is "Endangered" (IUCN Red List) in its natural habitat (Rio Grande do Sul, Brazil). The flower can be yellow, orange or red on different plants.
Many decades ago this was in the genus Notocactus, now combined with Parodia. N. ottonis and N. scopa were/are to be found in most amateur collections.
Fortunately, species of Parodia propagate easily from seed.
The stereo is crosseye.
Olympus EM-1, (aperture priority), Olympus 4/3 50mm f2 macro, 1/400 at f8 ISO 250, hand-held.