Can't find the text, but this is from the ATF FAQ page.
[url]http://www.atf.gov/firearms/faq/faq2.htm#o3[/url]
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 same 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)]
Notice that there is no requirement mentioned for a date. Post-ban Glock mags, for example are marked as "Restricted after 9/13/94" or something along those lines.