The description of Rebutia calliantha was first published in Sukkulentenkunde 2: p. 25, in 1948 by W. Bewerunge. The description is stated to be from material in cultivation owned by a Mrs. Winter in Frankfurt, from plants imported by Stümer or Marsoner together with Rebutia wessneriana (i.e. presumably from the same location), but without location data.

Here is the text of the description in Latin, as published, with my own English translation below. You should look up the original for further details including additional notes in German.

Rebutia calliantha Bew. spec. nov.

Oblonga, ad 15 cm alta et 6,5 cm crassa, viridis, rarius proliferans; tubercula 3 mm alta; areolae parvae, orbiculares, tomentosae, 6 mm differentes; aculei radiales et centrales vix differentes, albidi, tubo ovarioque rubra squamisque violaceis; petalibus carmineis spatulatis; pistillo roseo stigmatibusque albidis; filamentis rubris, anitherisque aureis; fructus, semina, patriaque ignota.

English translation

Cylindrical, up to 15cm high and 6.5cm thick, green, rarely branching, with 3mm high tubercles; areoles small, circular, felted, 6mm apart; central and radial spines alike, whitish; floral tube and ovary red with purplish scales; petals spatulate, carmine; pistil pink and stigmas whitish; stamens red with golden anthers; fruit, seed and habitat not known.