48 CFR 232.905: 232.905 Payment documentation and process.

(b)(1)(iii) For task and delivery orders numbered in accordance with FAR 4.1603 and 204.1603, the 13-character order number may serve as the contract number on invoices and receiving reports. The contract or agreement number under which the order was placed may be omitted from invoices and receiving reports. The contractor may choose to identify both the contract number and the 13-character order number on invoices and receiving reports. Task and delivery orders numbered with a four-position alpha-numeric call or order serial number shall include both the 13-position basic contract Procurement Instrument Identifier and the four-position order number.