9 CFR § 381.123 - Official inspection mark; official establishment number.
The immediate container of every inspected and passed poultry product shall bear:
(a) The official inspection legend; and
(b) The official establishment number of the official establishment in which the product was processed under inspection and placed as follows:
(1) Within the official inspection legend in the form required by subpart M of this part; or
(2) Outside the official inspection legend elsewhere on the exterior of the container or its labeling, e.g., the lid of a can, if shown in a prominent and legible manner in a size sufficient to insure easy visibility and recognition and accompanied by the prefix “P”; or
(3) Off the exterior of the container, e.g., on a metal clip used to close casings or bags, or on the back of a paper label of a canned product, or on other packaging or labeling in the container, e.g., on aluminum pans and trays placed within containers, when a statement of its location is printed contiguous to the official inspection legend, such as “Plant No. on Package Closure” or “Plant No. on Pan”, if shown in a prominent and legible manner in a size sufficient to ensure easy visibility and recognition; or
(4) On an insert label placed under a transparent covering if clearly visible and legible and accompanied by the prefix “P”.