(a) Presentation of documents. At the first port in the United States after transportation through Canada or Mexico under the provisions of this subpart, the carrier shall present to Customs the in-transit manifest or manifests for each loaded conveyance. For mixed ladings, that is, ladings made up of several shipments, the waybills shall be available at the port of return or discharge for use by Customs officers. For a railroad train for which a consolidated manifest was not used the conductor shall also present a train sheet showing the car numbers and initials.

(b) Vessels and rail shipments continuing in-transit movement—(1) Vessels. In the case of a vessel carrying in-transit merchandise, the master's copies of the in-transit or in-bond manifest covering the merchandise given final Customs release at that port shall be retained by Customs at that port and the manifests covering merchandise to be discharged at subsequent ports of arrival shall be returned to the master of the vessel for presentation to Customs at the next port.

(2) Rail shipments. An in-transit rail shipment arriving at an intermediate port of reentry or exit intended for further in-transit movement through Canada may be permitted to go forward under the accompanying in-transit manifest after verification by Customs that the manifest satisfactorily identifies the shipment.

(c) Checking and breaking of seals—(1) Checking seals. The Customs officer at the port of arrival shall check customs seals applied to the conveyance or compartment for unlocked or missing seals. Where the seals are unlocked or missing, the merchandise shall be treated as having been imported from the transited country.

(2) Breaking seals. In-bond seals shall be broken only by a Customs officer or by a person acting under the direction of a Customs officer. In-transit seals may be broken by any carrier's employee, or by a consignee at any time or place after the merchandise under such seals has been released by Customs.

(d) Proper manifest. In-transit merchandise shall not be released until proper in-transit manifests are received except that it may be treated as imported merchandise.

(e) Substitution of merchandise. Any instance of substitution of merchandise shall be reported to the Commissioner of Customs, and the merchandise shall be detained.