541 Subfield $a - Source of acquisition:

Subfield $a contains the name of the person(s) or organization that is the source of the material. If the source is an agent, the owner is recorded in subfield $f (Owner) .

Examples:

541 ##$cGift;$aLeavitt Hunt's daughter, Mrs. William E. Patterson;$d1947.

541 ##$aSource unknown.