Wow. Thanks [USER=4963]RafalFett[/USER]. Amazing catch. This is possibly very bad news, indeed.
The only hope for a counter argument is that the code was always 200-21076C and the 200 was simply left off in the earlier ad for space requirements, while at the same time, the numbers at the end of the board would need to be something more like WR20781 and do not have anything to do with a date. But that's a lot to ask for I think when you put all the pieces together like you just did.
If that is a date code on the board then the case for the whole thing being made in 1981 is almost beyond dispute. The chips are July 1980 and Jan 1981 so that would line up very well with a June 1981 board.
Wow, yes, I feel just like Grogu there. Holy moly. Perfect gif for this. lol