Cookies would seem to be an easy way to implement this, particularly given that this is in no wise intended to grant every AM a unique identity for all time. They can create an account for that. If a thread goes on for weeks, and for some reason AM-27 becomes AM-34, it's not a big deal. Similarly, if someone logs in from one browser to comment on a thread, and then logs in from a different browser to comment on that same thread, it's not a big deal for them to be both AM-27 and AM-24. Again, that's what accounts are for.
As for whether to use digits or some other identifier, I have no particular opinion. Digits would seem to be sufficient, but I'm willing to believe otherwise.