How can they be considered serial numbers??

Serial numbers are used to be able to tell one item in a group from others in the same group apart. Cars, aircraft, televisions, cell phones, etc have serial numbers - you can isolate to a particular unit. In my particular working environment - aircraft seats, galleys, etc have serial numbers - each individual seat, galley, etc. has a work order noting what materials or parts were used to produce that item, it's called tractability.

Sometimes slot cards, room keys have production run numbers (sometimes called "lot number") as they all used the same material from the same material lot. In Charles's case of a whole barrel full of dice with the same number each die is not an individual die but one of a multitude of die made from the same material at the same time. This lot then could be shipped to a customer and any addition orders for the same identical die could be account for by use of a different "lot number".

I hope I made myself clear, in my old age it's becoming a little more difficult to explain myself.

Larry