If you send me us complete serial number we can look it up to determine if it is pre or post ban. Only lower receivers shipped as complete rifles before the Federal Ban of 9-13-94 are pre ban.
Some States have Laws that do not allow the pre ban type firearms unless the are true pre ban models manufactured before the Federal Ban such as CT, MA, NJ, Puerto Rico and NY
The list at AR15.com is helpful but not so exact for all manufacturers. For example, for Armalite/Eagle, it indicates that slightly over S/N 30,000 was the Pre-Ban cut-off, but recommend that you call to verify becasue some Eagle lowers below the 30,000 range were determined to be POST ban by BATF. Armalite charges $30 plus postage ($4.95) for the "Letter of Authenticity".