The Armanen row was published by Guido von List in 1908. Its eighteen characters are his arrangement, and the honest description of any one of them separates the historical character from what he added.
The character behind it
Thorn corresponds to the Younger Futhark character whose Old Norse name was thurs, meaning giant or ogre, a name the Norwegian and Icelandic rune poems treat as thoroughly unpleasant. The Old English rune poem calls its equivalent character thorn and describes a sharp thorn that wounds anyone who grasps it.
What von List did with it
He took the second of those two names. Calling the character Thorn rather than Thurs is a choice between two genuinely attested traditions, and it is one of the few places where von List's naming has an actual manuscript behind it. The sound value, a dental fricative, is retained. The meaning he assigns within his eighteen belongs to his own system.
The framing, and the afterlife
Von List presented the eighteen as an ancient priestly alphabet recovered rather than composed, and set the count at eighteen to match the eighteen rune-charms of the Havamal in the Poetic Edda. That recovery claim is not accepted by historians of runology.
The row was afterwards adopted in volkisch circles and by National Socialist organisations in the 1930s and 1940s. This atlas records that reception rather than passing over it.