And with regard to wanting to avoid calibre... nobody's likely to be willing to reinvent the wheel for a handful of your out-of-date MOBIs. You don't have to marry calibre or anything; use it to fix all of your old MOBIs and then purge it from your computer forever if you like. If you don't have the resources to script your own solution, I'm not sure you should willingly turn your back on any existing solution that will eliminate your issue.
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