Something put those bytes in there (as they are not 0xFF) and it almost certainly has to be the factory wafer ATE.
I think they are *NOT* a UID because Atmel, now Microchip don't *GUARANTEE* that they will be unique. Obviously X,Y coordinates within a wafer will repeat, and over multiple batches the wafer number will repeat, so if the lot no. isn't guaranteed unique (i.e. due to limited code space of 6 alphanumerics, which probably also encodes the fab and line) they could reuse them, returning to the start when they run out of valid lot numbers for a particular fab/line. Also, its a management issue - if they have to restart the wafer ATE partway through a run, do they *ALWAYS* guarantee to preset the last wafer no. used correctly? It obviously benefits them for traceability to have a near-unique serial no. but I suspect the extra costs to publicly guarantee its a UID and make it so aren't insignificant.
If my suppositions are correct, the odds of finding a serial no. collision between current production genuine chips are vanishingly small, and although such collisions between current chips and old genuine chips are possible, your chances of finding one 'in the wild' are infinitesimal, mostly due to the small number of chips that will have survived in active use that are *NOT* in a dedicated application where the firmware will never be changed again.