Another Modesty fan sent this in to me:
The Robert McGinnis cover of the first Fawcett-Crest 'Modesty Blaise' paperback was also used, it seems, for a foreign paperback cover of a Henry Kane novel.
The Paperback Covers of Robert McGinnis, which provides an index of all of McGinnis' covers, makes no mention of any Henry Kane novels, so the art likely commissioned for the Modesty Blaise novel and then licensed to the Finnish publisher.
This translation was published in Finland in 1966; the original Kane book is titled Peter Chambers #13: Death on the Double (thanks, Sami S!). There's some information on author Henry Kane here, for anyone interested, and a bibliography of his works on this site.
Thanks to all the Modesty fans who contributed information!