One can engrave QR codes with a lasercutter. I've used them to provide a link to where to download the source files, for making more or to integrated the design

One can engrave QR codes with a lasercutter. I've used them to provide a link to where to download the source files, for making more or to integrated the design with other objects based on the CAD sources. This was primarily to be read by humans, but one can imagine the same structure readable by machines.