Thanks, knc1.
I guess my weekend won't be boring...
To answer your questions: I don't think I overwrote mmcblk0.
The (bad) ID it shows now is the one it had before any flashing was done.
EDIT:
While browsing the pw_5.2 IMG files, I noticed there are no libraries version 3.x.x, only 2.6.x...
My understanding is, this is related to the Linux version.
Could this mean, that the demo OS the Kindle was running retains some of its parts that are not replaced when flashing (dd) the mmcblk0p1 image file? And the demo OS was newer based on 3.x Linux...
If so, what could those remnants be?
What else needs to be flashed to take the Kindle back to the pw_5.2 days (OS-wise)?
Or, what other IMG can try to flash that wold be based on the same Linux version as the demo OS (3.x.x)?
Or am I digging in the wrong direction?