Sukiyaki Western Django, no code needed for Digital Copy - How?!?

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I just picked up Sukiyaki Western Django on Blu-ray, and was surprised to find a 2nd disc for the digital copy, since I hadn't known it was coming with one. Even more surprising was that there was no instruction sheet and no license key. You just put the disc in your computer, and it somehow gets a license key and informs you how many uses you have left.

How is this achieved? My understanding of pressed DVDs is that they should all be identical. I can't imagine any DVD manufacturer pressing these discs one at a time, updating the pressing master each time so a new key is included. But yet something must be different about each disc, otherwise there'd be no way to keep track of how many licenses any individual had used. Do DVD pressing plants have the capability to make small changes to each disc while doing a batch run from the same master?

Please enlighten me, this has thoroughly piqued my curiosity.