Much to my surprise I've been informed that there's also a Русский язык по методу Натура written by Oleg Objedof-Koefoed under the direction of Arthur Jensen. The
catalog of Copyright confirms it. I have never seen it.
If you search in Google Images for "Russisch nach der Naturmethode" you'll find a lot some images of the book (confirming it does exist), and that it was sold at
http://www.willhaben.at, the item is no longer available. They do show however that it was sold by the following company in Austria:
Humboldt Fernlehr Institut
Lothringerstraße 4-8
1041 Wien
According to the following
article, the Humboldt Fernlehr Institut ceased to exist as of 2017 and had been bought by the
Humboldt Matura-Schule. The course doesn't appear in their catalogues, I've written them an email anyway. I'll post any information they give me here, but I'd appreciate if anyone in Vienna could pay them a visit and conduct a more thorough interview, I'd be specially interested in getting in touch with the former owners of the Humboldt Fernlehr Institut, inquiring about the copyright status of the method and knowing what relationship they have with the other Nature Method courses.
And yet I fear that no amount of internet searching is going to be of much help locating any other of the missing methods, we have to begin asking around to our grandparents and their friends, someone somewhere must have at least a copy of them in their attic.