Rebuilding the iPhoto library on both the old and new Mac (by holding down Command and Option while starting iPhoto, etc) did not help.Attempting to run iPhoto Library Upgrader 1.1 (which 'prepares libraries from iPhoto 7 or earlier so they can be upgraded with the current version of iPhoto') on yet another copy of the original iPhoto Library returned 'Your library can open with the current version of iPhoto and does not need to be prepared with this tool.'.Deleted Photo Library and iPhoto Library, manually copied iPhoto Library from old Mac again, relaunched both iPhoto and Photos - same result.

Photos Library displayed only 1,794 photos (through September 2010), even though all photos were found in ~/Pictures/Photos Library.photoslibrary/Masters and ~/Pictures/Photos Library.photoslibrary/Previews.Imported data, including iPhoto Library, to new Mac running OS X 10.11.5 and Photos 1.5.Old Mac running OS X 10.6.8 with iPhoto 6 library containing 5,238 items.Missing photos after upgrading iPhoto 6 to iPhoto 9.6.1 or Photos 1.5 # Missing photos after upgrading iPhoto 6 to iPhoto 9.6.1 or Photos 1.5

What is iphoto 9.6.1