Final answer:
To handle SLIT items with unreadable or unknown MCC's, it is recommended to request MCC replacement from the appropriate authority.
Step-by-step explanation:
To handle SLIT items with MCC's that the ALN/SN have become unreadable or unknown, the recommended course of action would be to request MCC replacement. To handle SLIT items with unreadable or unknown MCC's, it is recommended to request MCC replacement from the appropriate authority.
This involves contacting the appropriate authority and explaining the situation so that they can provide you with new codes to replace the unreadable ones. Destroying the items immediately or placing them in quarantine would not address the issue, as it is the MCC codes that need to be resolved.