Leonie,
Thank you again!
The 2008 thumbnail folder appears to include only the normal monthly activity, but inspect files numbers in each (using command-i to look at numbers of items in each folder, in each month) I do indeed find one day in 2008 that includes multiple-thousands of thumbnails. I conclude that it must be the date I imported a huge iPhoto library.
That explains why I appear to have a thumbnail for each picture, going back to the first photo by date (2005), in spite of the (redundant) rebuild that wants to happen automatically with each launch of Aperture.
I intend to do your recommendation: to manually highlight all photos prior to 2008 and generate thumbnails.
However, while waiting (expecting a long delay before I received an answer--I was wrong about that--thanks again!), I highlighted those same photos, and did a "reprocess from masters". That will take me a long time.
Then I will try the generate thumbnails.
Then I will report back!
Even on a blazing fast machine--MBP retina, with 750G flash drive, and Masters stored on 6TB raid via thunderbolt-eSata--these things take time! And I'm mindful to try (to change) *one thing at a time* to see if it fixed the problem, so that I can benefit from (and share with others) a simple cause-and-effect confidence that I fixed a problem.
FYI: I have been stalled in my editing now for almost 48 hours. Two days ago, my formerly reliable Aperture editing procedure began to freeze (spinning beach ball) while I tried so fairly routine edits on pictures. I'm not sure the editing problem still exists, because I have allowed myself to become side-tracked by this rebuild issue, thinking I might solve my editing problem in the process. However, the two might be completely unrelated.
I'm still trying to figure out what I changed (if anything) that might have caused the spinning beach ball. I can think of only one thing, and it has to do with previews--but I had better put that aside for now, until I can move ahead with this thumbnail issue (or just ignore it, if I can't fix it, as I was doing before the problem occurred, without apparently affecting the editing efficiency, prior to 48 hours ago).
I have customers waiting patiently, but Aperture is stalling my ability to generate income.
Were it not for this community--and particularly your help over the last 24 hours--I would be much more anxious!
Cheers,
Patrick