I don't have insider knowledge... But I could believe that there is no per-die serial number unless the customer wants one. It would be extra manufacturing

I don't have insider knowledge... But I could believe that there is no per-die serial number unless the customer wants one. It would be extra manufacturing steps, and require integration with the customers design. Due to the way lithography works, it isn't easy to make each die different. The usual way to put serial numbers into chips is with efuses, but not all chips have any efuses at all, and it would require collaboration with the customer to design a way that they be programmed and read (probably on a JTAG chain).