Restrict the use of 501 field to two or more works "contained in the physical item at the time of publishing, release, issue, or execution" (cf. MARC21 documentation, 501 field). This is often referred to as Nonce collections, where volumes made up of previously published materials are reissued, usually with a collective title page but not always. It is Houghton policy to create a single bibliographic record for the collection (with a contents note). Curatorial requests to create individual analytical records should be discussed with the Head of Rare Book Section.
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For works bound together subsequent to publication ("bound-with"), place this information in a 562 with accompanying 977 fields field in holdings. (see Section V.3.).