Extra-illustration (or "Grangerizing") was a practice among book collectors from the late-18th to early-20th centuries. Collectors disbound printed works, inserted additional material related to the work's content, and then rebound the whole — thus providing "extra" illustration to the original work. Added material of interest to theater historians can include portrait prints, manuscripts, and playbills.
For example, in 1867 T.H. Morrell published a 4-volume edition of Joseph Ireland's Records of the New York Stage from 1750 to 1860. Theater manager Augustin Daly had his copy disassembled and rebound with a wealth of material he had collected over the years, extending it to 33 volumes. Opposite page 525 of Ireland's text, which mentions a production of Open House, Daly tipped in a playbill from that performance:
Searching for Extra-Illustrated Works
You can search HOLLIS for Extra-Illustrated works by going to the Advanced Search screen. Select Library Catalog from the Search for: options, change Keywords Anywhere to Form/Genre in the dropdown menu next to the first search box, and enter Extra-illustrated copies (Provenance):
You can then use the second search box to search for a name, production title, or other term relevant to your research.
Some extra-illustrated copies at Harvard have descriptions of the additional material that was tipped in by their creator. That description may take the form of a finding aid. For that reason it can be useful to search Harvard's finding aid search tool, HOLLIS for Archival Discovery, for terms relevant to your project.
The Harvard Theatre Collection holds a large number of extra-illustrated works. The bulk of the material tipped into these is described in a physical card catalog, which has been scanned. Information on this collection, and how to search the scanned cards, can be found in another online guide.