Joseph Smith writes the Book of Mormon preface , with reference for the first time to an exact page count for "116 pages" that were lost. Because the printer's manuscript is 116 pages for First Nephi through Words of Mormon, the replacement portion, it can be assumed that the number 116 came from the printer's manuscript rather than from the original manuscript. It was an approximation, in other words--an assumption that the original manuscript occupied about the same number of pages for the same period covered. It can be further assumed that Oliver Cowdery would have copied at least 116 pages of the manuscript for the printer before the preface was written. Cowdery probably began copying the manuscript near the beginning of July and apparently reached Alma 36 by 6 November 1829 . Averaging the number of pages per day, he would have copied 116 pages by about the beginning of August. Thus, since the first signature included the Preface , Joseph Smith probably wrote the preface shortly before the printer began setting type in mid- or late-August.
[Vogel, Dan, Early Mormon Documents, Appendix B: Chronology, 1771-1831, http://amzn.to/T5nY8w]