After some intensive searching I recently have obtained a box of this exclusive grain from a source in Germany who got it from a farmer who has been producing it for about 8-10 years with original seeds from Sepp. They call it Secale multicaule or Waldstaudenkorn.
This is what I found out:
Sepp's mysterious rye is the species 'Secale multicaule
Kühn et al. 1974'. Taxonomically this is not correct and derives from Secale cereale
Metzg. ex Alef., Landw. Fl.: 338 (1866).
This is nowadays not an accepted name. It is simply called Secale cereale
but that's also stupid in my opinion because that's the name for all ordinairy rye species. So Joseph Holzer was right when he said "it´s a variety of Secale cereale"
The common use though is Secale cereale
In German it's mostly called Waldstaudenkorn
(Wald = forest, Staude = perennial herb, Korn = cereal) or Johannisroggen
(St. John's Rye, because it is sown around St. John's Day).
(Ones I knew the name I could even find it on the German Wikipedia. à https://de.wikipedia.org/wiki/Waldstaudenroggen
Other names: Urroggen, Sibirisches Urgetreide, Sibirischer Roggen, Sibirisches Urkorn or Waldstaudenroggen.
Sepp Holzer calls this ancient grain (=Urkorn) 'The Grandmother of all rye' (Großmutter de Roggens). It has been used in Slash-and-burn farming probably for thousands of years and it was sown without ploughing.
Also hunters used it in a mix for small mountain vegetable plots in the woods.
Years ago a german farmer obtained seeds from Holzer and started cultivating it as a pioneer project and he is now growing it locally in 'small' amounts (2,5 acres).
There are several farmers in Germany growing it these days so it is on the march. It is not certified for seed selling, allthough it is allright selling it for making bread as far as I understand. It is hard to get it due to the small production.
Some more background information and growing instructions from the farmer:
Source: Pflanzebau & Technik - http://www.permakulturberatung.de/PDF/Bioland%20Zeitung%2008Hofer.pdf
He prefers to sow in the spring and than mows it twice the first year (or let sheep graze on it). Then the plants get strong and the next year he harvests in autumn. This increases the harvest with 30%.
Other farmers mow it as much as three times before letting it come to full growth. After harvesting the plants die.
Sepp's own instructions can be found in The rebel farmer (page 270 of "Der Agrar-Rebell', I only have the german version)
Well, that's all I found out so far…
My personal rye stock: