They don't take pictures of the mail for fun or for tracking, it's purely for sorting. You want all the mail in a letter carriers truck to go into the truck in the order it's going to be delivered in for efficiency.

So the system was in one of two modes, destination address coding or return address coding. Which mode it was in was determined by the mail processing plant itself.

Also, it only photographed letters that did not have a presorted barcode printed on them. If it already had a barcode it just got sorted immediately.

I'm not sure what there really is to tap into here.