I'd just like to point out again, that on import of a .mobi/.azw/.azw3 book, if the exth 113 field (Amazon uses this for ASIN) contains anything, it will be added to the metadata
identifiers field in calibre.
I assume any .mobi/.azw/.azw3 books that are being imported are from Amazon, and anyways, the field should not typically contain anything otherwise, so if there is an
Code:
{identifiers:select(mobi-asin)}
it is almost certainly purchased from Amazon.