@barsanuphe: It's in fact the original format used by the Collections feature on Kindles before FW 5.x came along
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The hash is generated (in this instance, by the plugin) from the metadata of the file, it follows the format '#ASIN^cdeType' if an ASIN is set [valid, like in my Apps exemple, or invalid [Calibre generated], like in the second collection. If the ASIN isn't set, it's formatted as '*hash', where hash is the md5 of the absolute path of the file (using /mnt/us as the userstore root).
As for the cdeType, I *think* it's mandatory for a file to be valid/collectionable, and if it isn't, I'm assuming that defaults to EBOK or PDOC.
Anyway, the plugin handles that aspect to make sure that what's end up in the json is sane.
On FW 5.x, in the SQLite db, those are stored in two distinct fields, as p_cdeKey & p_cdeType in the Entries table, and i_member_cde_type & i_member_cde_key in the Collections table, with the notable difference that the '#' is *NOT* present anymore in front of the ASIN (the '*' of the legacy hashes stays, though).
TL;DR: You can get the db uuid of an item from the db w/ these
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