Hi,
Be aware that the new KF8 mobi ebooks as generated by the latest Kindlegen can have two different mobi headers and two different EXTH metadata regions all within the same file. So you may end up editing the exth of the old mobi version 6 part, which will be ignored on the Kindle Fire in favour of the second mobi 8 version which will have its own metadata.
You can play with the DumpMobiHeader.py program to see all of the metadata and see if you have one of those new dual KF8 ebooks.
Here is the link in case you want to try it:
https://www.mobileread.com/forums/sho...63&postcount=8
This program could be easily extended to properly change/ write any EXTH in either or both headers if that would prove useful to anyone.
KevinH