Three half-leaves of the final manuscript of Principia Mathematica have come to light in the Bertrand Russell Archives. They were originally tucked in Russell's own copy but avoided archival notice because their versos had been employed for an index of propositions used in theorem *350·62. The leaves form the whole of a folio 152 and the top half of 153 and include *336·51 through part of *336·52, on pages 400–1 of Volume III. Markings by the Cambridge University Press add to our knowledge of the typesetting and proofreading of PM and give some indication of the fate of the remainder of the approximately 5–6,000 manuscript leaves, of which only one had been known to have survived.
Linsky, Bernard and Blackwell, Kenneth
"New Manuscript Leaves and the Printing of the First Edition of Principia Mathematica,"
Russell: the Journal of Bertrand Russell Studies:
2, Article 5.
Available at: http://digitalcommons.mcmaster.ca/russelljournal/vol25/iss2/5