Sorry, page 147 of Aqualog: Freshwater Rays, P40 Potamotrygon schroederi, pic looks exactly like yours, I don't know what to say. Closer examination of the book I find that nomenclature has a long way to go, to say the least!
that brown ray is "potamotrygon sp. peru" they are exported as common ray, hystrix and so on.
Some people call theirs "peru schroederi/flower" or "sasha ray"
But for me it is a undescribed peruvian species/natural hybrid.
Ask vendors here that imported "raya comun" or "raya hystrix" they must have got some looking like that.