This article might be helpful:
https://www.radiomuseum.org/forum/be...r_tetrode.html
It does include graphs that show the normal tetrode “kink”.
I understand that the name “kinkless tetrode” was coined by Osram, who then used the “KT” prefix for its valves of this type, of which the KT66 is probably best known.
Cheers,