Fender sheet metal Date Code?
 

6A11976.

June something 1976?

that is not a date code...
The fender and hood dates are typically only a number corresponding with the week of the year.... 53 on the hood is because of how weeks fall.

who or where in the process was the date/week stamped on the sheet metal. And what was it's reason for doing it?

By the stamping plant.
For traceability.

You can see accident replacement too.