The Full Form of FSN is Federal Stock Number.

The Federal Stock Number (FSN) was the codification system used by the U.S. federal government from 1955 to 1974. 

It was 11 digits long. The first four digits were the Federal Supply Classification Group (FSCG) code. This relates the item to the Federal Supply Group (FSG; digits 1 & 2) and Federal Supply Classification (FSC; digits 3 & 4) of similar items that it belongs to. The next seven digits were the unique item’s serial number, or Federal Item Identification Number (FIIN).

It was designed to simplify and standardize the cataloging of common items in use by the Department of Defense. It replaced the earlier hodgepodge of cataloging systems in use. The Standard Nomenclature List (SNL) was created by the US Army Ordnance Corps in 1930 and was used until 1958. The Ammunition Identification Code (AIC) was a subset of the SNL concerning munitions and explosives that was used from January, 1942 to 1958. The SNL and AIC were listed in the Ordnance Supply Catalog, which was managed by the US Army Ordnance Corps. The Quartermaster Stock Number (QSN), which codified items by government contract number, was created by the Treasury Department in 1943; it was used by the US Army Quartermaster Corps officially from 1946 to 1956. The Medical Departments of the Army, Navy and Air Force each had their own systems as well.