Rebutia marsoneri was first published in Kakteenkunde (1937), pp. 1, 2 & 4, by E. Werdermann.

Included was a black-and-white photograph of a small grafted cutting with long, very dense spines; which bears little resemblance to the plants we know from cultivation.

Here is the text of the description in Latin as published, together with my own English translation below. You should look up the original for extensive further notes in German, and the photograph.

Rebutia Marsoneri Werd. nov. spec.

Simplex (an semper?), depresso-globosa, vertice manifeste umbilicata, mamillis perhumilibus, areolis parvis atque breviter tomentosis, acuteis setiformibus ca. 30—35, in parte inferiore areolarum juvenilium suis brevioribus atque albidis, in parte superiore duplo vel triplo longioribus atque saepissime fuscescenti-ferruginei; floribus areolis infimis corporis exortis, ovario tuboque gracili squamis sparsis in axillis typice glabris obsitis, phyllis perigonii interioribus aureo-luteis, filamentis pallidioribus, stylo filiformi pallideque tuteo, stigmaiibus 4 albidis stamina longissima manifeste superante.

English translation

Rebutia Marsoneri Werd. nov. spec.

Single-bodied (always?), flattened spherical, the tops clearly dimpled, tubercles not very prominent, areoles small with a small amount of wool, ca. 30 – 35 bristly spines, shorter and whitish in the lower half of the areole, twice or three times as long in the upper half, frequently darkening to rusty brown on younger parts; flowers emerging from low in the body, ovary and slim tube sparsely covered with scales, typically hairless in the axils, inner petals golden-yellow, stamens paler, style hairlike, all pale, with 4 white stigmas rising above the stamens.