[ariel]

Well, the earliest "tunkous" are seen on the nomadic swords dating to at least 8-9 centuries. Subsequent tunkou-like elements are seen on a multitude of blades , all coming from the areas dominated by, or at least in contact with nomadic cultures. Their forms varied : from the " along the edge" in early examples to the "along the spine" in the latter ones, from massive plates to purely symbolic, decorative koftgari or incised outlines, from traditional triangular to "shell-like" in North African yataghans etc. Ottomans were not the originators of tunkou: they got it from their Seljuk and Oghuz ancestors, but due to their exclusive Western location, they were a vehicle of spreading it over the Mediterranean basin. Seljuk Empire had its epicenter in modern Iran, Indian Moghuls came from Central Asia etc.
The earliest European one I know (1321AD)can be seen on the fresco of St. Nikita in the Gracanica church in Serbia, perhaps as a result of Batu Khan invasion half a century earlier. I may go on a limb here, but the so-called Indian ricasso might be a direct descendant of the archaic Nomadic ones.
Both Khudyakov and Phillip Tom commented on their original purpose, and suggested purely utilitarian mechanical reasons: plugging the mouth of the scabbard, isolating the edge, or just secure seating of blades within their scabbards. Phillip Tom thinks that Japanese Habaki is unrelated to tunkou, but looking at the origin of Japanese people and their contacts with Koreans, Chinese and Mongols, this assertion may be modified.