I'm not sure what Black Mark's deal is. They're the only label I know of that frequently includes two different bar codes on one physical release. On a few albums I have, they even note that the EAN is for the European market and the UPC is for the U.S. I assume it's a cost saving measure from when they were doing their own distribution in America. In the early '90s they sometimes had small quantities pressed by an North American distributor, which were virtually identical to the European releases (including the same barcode) except for the addition of a logo and "Made in Canada" note. Towards the mid '90s you see two barcodes on most releases, which continues today (though I do have a couple more recent reissues of theirs with just one barcode). It's goofy.
Psychedelic Africa's second barcode is definitely an ISBN. It's explicitly identified as such. It didn't come with a book, so I assume it's for the liner notes.
I think allowing for multiple barcodes (and clearly explaining when it's appropriate to add them, i.e. not for barcode stickers applied over printed barcodes, not just a reprint with a new barcode, but only if there are multiple barcodes printed directly on one physical release) would actually clear up releases because it would enable the merging of legacy releases for which both barcodes were added, producing two NGS releases for one physical release. See this example, which is one physical release duplicated because of multiple barcodes thanks to Black Mark's weirdness: http://musicbrainz.org/release-group/89 … 2025f3123a
I think it would make things look cleaner overall and help limit future duplications.