For what it's worth--and in contradiction my earlier post--OED defines "folio" [A.I.1.] as, "A leaf of paper, parchment, etc. (either loose as one of a series, or in a bound volume) which is numbered only on the front", not "A leaf of paper ... that has text only on the front." (http://www.oed.com/view/Entry/72529 subs. req.)
Not that catalogers have to stick to the dictionary, even such a fine dictionary as the OED. But I found it interesting.
Acquisitions, Metadata and Enterprise Systems