535 Subfield $g - Repository location code:

Subfield $g contains a two- or three-character MARC code for the country of the repository holding originals or duplicates of the material. The source of the repository location code is MARC Code List for Countries that is maintained by the Library of Congress.

Example:

535 2#$3Harrison papers$aNeils Bohr Library, Center for History of Physics, American Institute of Physics;$b335 East 45th Street, New York, NY$gnyu