With reference to the CBP Form 7501 Instructions found here, block 24 is reserved for the Employer Identification Number (e.g., tax ID), Social Security Number, or CBP Assigned Number of the individual or firm to whom refunds, bills, or notices of extension or suspension of liquidation are to be sent.

While this appears to be the appropriate place to enter an importer's internal reference or purchase order number, these instructions explicitly state "do not use this block for any other information."