One potential explanation for the ascendance ( not the appearance) of the " inverted tunkou" may be purely artistic.
At some stage of the game ( my guess 14-15 century), in the Islamic areal tunkou lost its engineering meaning and became purely decorative. Inscriptions on the blade became popular. Stamps and cartouches were far too small to accomodate a dedication, a prayer or even a motto. Necessarily, they had to be oriented longitudinally. Also, they had to utilize a " less-working" part of the blade, the lower quarter or so, adjacent to the handle/ handguard. On top of that, placing them along the edge would mean an inevitable loss of the sacral inscription as a result of repeat sharpening. The solution was simple: place them along the spine. And here, cutlers could combine the above practical points with the existing fashion of tunkou: short segment of decoration occupying the entire width of the blade adjacent to the handguard and a long inscription along the spine. A quick example is shown.
This was a homage to the traditional tunkou, that utilized the " upside down" pattern. From that point on the majority of single-edged Islamic blades were decorated in that manner and the "classical" tunkou simply vanished.
Of course, this does not explain the initial appearance of the inverted tunkou, but perhaps it explains the later popularity of it.
Just a thought...
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