Huh. I thought it was using the sha1 of the absolute path, not a seemingly random increasing int... (Keep in mind that I never really looked at how the Touch FW behaved, and I mostly work with KF8, so thanks for the details ;-)).
EDIT: Yep, found a few thumbnail_<int>.jpg files on my devices. Never bothered to check that the number changed each time the same book was touched

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