@KevinH: Thanks for the details! (Sorry for the delayed response, I was away for a week

).
Nope, no idea on EXTH 131, but I think that's the one where I usually find an UINT32_MAX (4294967295) on AZW3 files from Amazon...
Yup, that makes sense for EXTH 525, I was mostly going from the log ouputs, which might not be related to this exact field

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I'm not sure I actually have something with EXTH 528 set (well, probably, since I found mentions of it it my logs, but I'll have to check)...