The ballad book of John Jacob Niles / with a new introduction by Ron Pen ; decorations by William Barss. (OCLC #904561492)

RDA 3.4.1.7.5 on recording extent of online resources says that when the online resource has a print counterpart, its extent should also be specified. This resource is a digized score, so we look at (RDA 3.4.1.7.5) which covers extent of scores. The print score has extent:

1 score (xxvi, 370 pages)

so the online version has extent:

1 online resource (1 score (xxvi, 370 pages))