Commercial MOBI (and AZW) ebooks include metadata identifying the ebook store, for example (via mobi2mobi):
Code:
EXTH item: 1 - drm_server_id - 9 - EBOOKBASE
EXTH item: 2 - drm_commerce_id - 11 - AMAZON_1110
EXTH item: 3 - drm_ebookbase_book_id - 6 - 251848
EXTH item: 100 - Author - 15 - Moon, Elizabeth
EXTH item: 113 - ASIN - 10 - B0030DHPAC
This isn't changed by mobidedrm.py, so it might be what Amazon it using to identify its ebooks.
If you run the ebook though Calibre (mobi to mobi), then this metadata will be removed.