The only valid use of the autonumber field is to generate unique record IDs.
Any other use of it will lead to troubles sooner or later. The sick thing
is that MS has some help on the support site that will help you manipulate
autonumbers in certain ways. Some of the MVPs have generated some neat
solutions, too. However, the righteous rule is: If you are trying to
generate a number that humans will see and/or attempt to understand then you
should generate and manage that number yourself. Check out "Autonumber" at www.mvps.org/access