Mr. 47's Barcode Tattoo
47s Barcode as based on images from Dr. Ort-Meyer's Journal. Barcode is in Code 39, according to the journal the barcode was added to the tattoo in 1975, Code 39 was developed in 1974.

The tattoo on the back of the protagonist's head 640509-040147 is actually registered as a product of May Audio Marketing Inc. It is unknown if there is actually any connection between the company and the game, but the product's description is listed as Mr.47. The barcode's function doubles as a security measure; Dr. Ort-Meyer placed scanners outside of his room to prevent anyone but his favored clones from entering. But 47 simply scanned the barcode off the corpse of another clone, sealing the Doctor's fate.