Paul writes:

I'd like to know how they put the serial number into the copy.

Most likely by modulating the Least Significant Bit of the color intensity in some (or all) color-filled areas of the image. A set of sync-bits, serial number, and some more sync bits. Repeat every line, shifted by 1 bit, and you'll get a pattern that is hard to differentiate from the background unless you _know_ what to look for. I wouldn't be surprised if there isn't a time and date stamp as well, plus batch # info.