It’s somewhat dubious whether the correct spelling be “Isakoff” or “Isakof.” In their catalogs, for some instances, Lambert and Mühle opt for “Isakoff,” Soupert & Notting for “Isakof.” The Rosenlexikon goes for “Isakoff.” It can be found either way in various periodicals of the time, some indeed inconsistently with themselves (Journal des Roses, Rosen-Zeitung, etc.). More recently, we see Meilland (La Vie en Roses, p. 210) writing "Isakoff." If we tally up actual occurrences, as far as we can from the available materials, "Isakoff" seems to have a slight numerical advantage. And yet, we need to keep in mind that truth is not subject to numerical superiority. On the theory that the notes of release of varieties were most often written by their breeder/introducers, who presumably scrutinized the text they were submitting carefully and would have raised a fuss that a note of correction be included in future issues had there been any error, we see it as ‘Marie Isakof’ in Dubreuil’s note of introduction at JR25/162, with no subsequent correction, and so perhaps may take that as our ultimate authority: 'Marie Isakof', which is how I will have it in my upcoming much updated new edition of The Old Rose Adventurer.