I'm with the two posters currently below me. When the factory runs out of the proper parts, they can do some "counterfeit" or sub-quality runs with near matche

I'm with the two posters currently below me. When the factory runs out of the proper parts, they can do some "counterfeit" or sub-quality runs with near matches and most features will probably still work under most conditions. The simplest example I can think of would be components value such as capacitors and resistors. If a design uses an uncommon part value (like 5.0uF) and the factory runs out, they can build with a common value (like 4.7uF) and come very close. This is most likely to happen at the end of a production run. There aren't enough parts leftover for the actual quality device, but you can make substitutions and get some more near-quality units out instead of throwing out all of the rest of the good parts. Another example I could foresee from the OP is that different revs of firmware were available at different times. e.g. a bug was discovered after the ROMs were masked or flashed and that run of chips should have been destroyed but weren't. (Hell, they might still be upgradeable!) Ironically, the serial number was possibly intended to distinguish the official runs from sub-quality runs... The "counterfeiters" might not have been trying to fool everyone, but instead, to distinguish between official and ghost shifts. Welcome to the new age! :)