I know what Michael is thinking, if it's electro-penciled on the slide it's probably an evidence or case number from a criminal case and the LA indicates the jurisdiction. A lot of police departments will do that because a hang tag could be easily lost.
Interesting. I've had many guns that were owned and issued by law enforcement agencies and some of them did have a secondary number on them that was engraved at the factory in accordance to the contract for the agency's order. This is not the serial number of the gun but it is a property or inventory number assigned by the agency. This might be the case here.