530 Subfield $b - Availability source:

Subfield $b contains the availability source which is defined as the organizational unit or vendor from which the additional physical form may be acquired.

Example:

530 ##$aPhotoreproduced facsimile version;$bPublished as Dudley, Cuthbert, ed., The Novel of Lord Ethelbert of Waxlot (Oxford University Press, 1973).