A magazine which is not marked as stated below is presumed to be of preban manufacture. It has been reported that some postban magazines were manufactured with just a date and were not marked restricted as required by law.
A preban magazine may be used in any firearm under federal law and if not restricted by state or local law.
From the ATF FAQ:
(O15) What markings must appear on large capacity ammunition feeding devices
manufactured after September 13, 1994?
Persons who import large capacity ammunition feeding devices manufactured after September 13, 1994, or manufacture large capacity
ammunition feeding devices must legibly identify each device imported or manufactured by serial number and other prescribed markings. The same serial number may be used for all devices manufactured or imported. Such devices
must also be marked "RESTRICTED LAW ENFORCEMENT/GOVERNMENT USE ONLY" or, in
the case of devices manufactured for export, "FOR EXPORT ONLY." Domestically
made devices must also be marked with the name, city and State of the manufacturer. Imported devices must be marked with the name of the
manufacturer, country of origin, and name, city and State of the importer. Persons who manufacture or import metallic links for use in the assembly of belted ammunition are only required to place the prescribed identification
marks on the containers used for packaging the links.
[18 U.S.C. 923(i), 27 CFR 178.92(c)]