Yeah, there was a lot of confusion because of that article, and it was dead wrong.
Here's how your Apple serial works:
First 2 Digits: Factory it was assembled at
Next 1 Digit: Year
Next 2 Digits: Month manufactured
Remained: Your unique serial.
For example: W8619XJ42U
W8: Shanghai Factory
6: Built 2006
19: Built 19th week
XJ42U: Your Mac's serial number
So, people who thought that W8612... or W8613... meant revisions of different motherboards, but the only difference was one was built the following week.