The reason the DIDX jumped to my attention is that there is another phenomena /common practice in CDs whereby the album will be dressed up in not only new packaging, but assigned new catalogue numbers and barcodes and sold as a new issue, but when you look to the matrix / inner ring, it is evident that they have just used the old matrix / issue, and given it new clothes...
...And so, what used to be also the catalogue number leaves a residual imprint on the "new issue" in the shape of catalogue number now being only tucked away as a matrix.
Another correction I did at the same time as this has this very thing:
...Look at the inner ring of the Simon and Garfunkel: Bridge Over Troubled Water on Qaud's images... there you'll see a "CDCBS" prefix number, which was the catalogue number system used by CBS in the early days of their CDs, and while I've not looked for the earliest version of that particular album on CD, I'd bet that the catalogue number for that matches exactly the number to be found on Quads newer version's inner ring.
Cheating perhaps? ... dressing up old rope as new?
Anyway, I just wondered if the same thing was happening here with the DIDX... possibly an early issue which has that as a catalogue number, and so wondered if it ought to be included as a variant here.