Created after World War II by the U.S. military to standardize the inventory numbering systems of supplies. actual time now Prior to the National Stock Number each branch of the Military adhered to their own unique numbering system. This made it difficult to locate and share supplies.

National Stock Number (NSN) is a 13-digit numeric code, identifying all the standardized material items of supply as they have been recognized by all NATO countries including United States Department of Defense (DOD).

The 13-digit NSN # 3120-01-152-2039 is comprised of

First 2 digits – Federal Supply Group (FSG) number (31 in case of NSN 3120-01-152-2039)
First 4 digits – Federal Supply Class (FSC) number (3120 in case of NSN 3120-01-152-2039)
Digit 5-6 – Country Code (01 in the case of NSN 3120-01-152-2039) for the country assigning the number
Last 9 digits – National Item Identification Number (NIIN) (01-152-2039 in case of 3120-01-152-2039)