Forget what I said about the S/N's being skewed by 1 S/N. Sometimes they are, sometimes they aren't. Don't make this more confusing then necessary, my head will explode
Just make a notation in your S/N book that the data may or may not be correct for that S/N range. If you have a S/N in that range, rest assured that the order book and stock book entries will be correct and your letter will be correct.