Which letter should be added to a message key code for a stolen gun record held for latent fingerprint examination?

The letter that should be added to a message key code for a stolen gun record held for latent fingerprint examination is 'P'. This designation indicates that a firearm is being analyzed for potential fingerprints, which is a critical aspect of forensic investigation. The addition of this letter effectively communicates to law enforcement agencies and databases that the firearm requires further examination, such as identifying potential evidence that could lead to solving a crime.

In the context of this procedure, the other letters correspond to different types of records or statuses that do not specifically indicate that a latent fingerprint examination is pending. Thus, the 'P' serves as a clear marker for a stolen gun record needing attention for fingerprint analysis, ensuring that all relevant parties are informed of its current status and the necessary investigative steps.