For the EXTH header it seems that 400-499 is "application binary" and 500-599 is "application string". So I think these headers are Kindle specific.
The flags to use in mobi2mobi to test different things are probably:
Code:
--booktype number
--exthtype number --exthdata number
--delexthtype number
I can also say that I read the decompiled Java code for the Kindle but did not find how a collection/subfolder was detected. Maybe somebody else that have read the code can help?