5331-00-720-2139 (4K1-25)

NSN 5331-00-720-2139 belongs to Federal Supply Class 5331 which is 5331 - O-Rings. This FSC is part of the federal supply catalog maintained by the U.S. Department of Defense and is used to standardize the procurement and cataloging of military and government components across all branches of the armed forces and federal agencies.

Traceability is a core part of our sourcing process. Depending on the part and its supply chain origin, we can provide Certificates of Conformance (C of C), manufacturer packing slips, date codes, lot codes, and sourcing documentation that traces the item back through the distribution chain. For government and defense procurement, we understand that documentation requirements can be strict if your purchase order or contract specifies particular traceability requirements, include them in your RFQ and we will confirm our ability to meet them before order confirmation.

Yes. NSN 5331-00-720-2139 is also referenced by manufacturer part number 4K125, where 4K125 is the original equipment manufacturer designation for this item. When sourcing this part, buyers may encounter either the NSN 5331-00-720-2139 or the MPN 4K125 depending on the procurement system or supplier being used. NSN-PARTS.COM accepts quote requests using either the NSN or the manufacturer part number. For a full list of alternate part numbers associated with this NSN, see below.

Every NSN includes a two-digit National Codification Bureau (NCB) code in positions 5 and 6 of the full 13-digit number. That code identifies which country originally cataloged the item through the NATO codification system. You can find the NCB code for this part listed in the product description above. Use the table below to identify the originating country and its NATO membership status.

For this NSN, the NCB code is 00, which means it was originally cataloged by USA (NATO). Use the table below for a full reference of all NCB codes and their associated countries.

When a part is not immediately available through standard distribution channels, our sourcing team conducts a multi-channel search that includes authorized distributor networks, government surplus and DLA excess inventory, independent broker networks, and manufacturer direct channels where applicable. Every potential source is evaluated for authenticity and traceability before we present it as a viable option. We do not source from unvetted or unknown suppliers, and we will never offer parts we cannot document. If we locate stock, we provide full sourcing details with your quote so your procurement and quality teams can make an informed decision before committing to an order.

We make every effort to ensure full chain-of-custody traceability for every part we supply. Our preferred sourcing path is always manufacturer-direct or through authorized distribution channels, which provides the strongest traceability and the lowest counterfeit risk. When parts must be sourced through independent or excess channels; as is often the case for obsolete or long-discontinued components, we require sourcing documentation at every link in the chain and apply additional scrutiny to confirm authenticity. Parts that cannot be adequately documented are not offered for sale. If your program has specific traceability requirements such as QPL, QML, or QSLD compliance, please include those details in your RFQ so we can confirm sourcing eligibility upfront.