What made you decide to check this all out?
Logical123 made a
thread about NES console variants and I told him there were differences in the number of screws holding down the shield, as that was one I knew of off the top of my head.
And then I happened to be taking apart several NES systems to clean and noticed the different other stuff on them. I tried to find a pattern relating the serial # to the copyright year or board #. But I didn't have enough to find a pattern, so I made a post in Logical's thread and eventually it got moved to a separate thread.
And thanks to those who have contributed to this, it's much appreciated.