Question About Serial #'s

[Saldo]

Do all Atari systems have a serial # sticker on the bottom of them that match the box they came in? I just got a Vader system in the box that has no serial number sticker on the system. The box and internal packing have matching #'s but nothing on the unit. Doesn't even look like there was one because there is no place on the unit that seems to have residual stickyness to it. I'm thinking either it came off and the owner cleaned it off real good, or something else real sinister.......