Don, That has been my theory for many years as well. I would say that each lot of chips has a specific code built into it as well to mark the production date.

Note the metal centers have a date on one side. "377" is March 1977 for example.

I still suggest that the location on the chip has something to do with the month and the year.

It could be as simple as the month being the position according to a clock. 1-12 and the rotation or quadrant of the design as the year.

I'm not sure this works, and in order to test I would get a number of chips recently produced and known to be new on the tables and see if there is some sort of relationship.