The batch # is stamped on one of the long sides of the outside of the box.
Nobody is perfect I guess, but every time I see a thousand jigsaw puzzle pieces poured out of a plastic bag I am amazed at how rarely a customer tells us he/she is missing a piece. Imagine having to make sure every single piece goes into every single bag before the puzzle leaves the plant. When the bag is opened and dumped on the coffee table every single piece has to be rescued from the cat, the floor, and the cuff of uncle Jack's pants. Once in a rare while we do hear from a customer who has the dreaded 999 piece problem.
It would be mean to send a new 1000 piece puzzle to start over, so we do our best to find the missing piece from some cannibalized puzzles we keep around here. We get the production batch number and a picture of the spot where the piece needs to fit. If we have an assembled puzzle available we can usually find the orphan in a few minutes. The real fun is when we don't have an assembled puzzle available. In that case we spread all the pieces out hoping the color will be obvious. If that doesn't work we take the right side up pieces off the table leaving about 500 blank blue piece on the table. My daughter can look at shapes and pick out the right piece in minutes. I'm glad it happens so rarely. It's still fun.