Colt ar 15 a3 serial number prefex lbd what does it stand for?

The LBDXXXXX serial number was designated for the Colt AR-15A3 Tactical Carbine (AR6721) post ban. Pre-ban version had the BDXXXXXX serial numbers. (there less than 200 of those released). Colt started using the LBD serial numbers late 1995/ early 1996.

The "L" was soley for the Law Enforcement Restricted models that had the "Restricted, Military/Government Law Enforcement/Export Use Only" roll marks on the magazine well.