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One possibilty is that your server was down during a google crawl ang gbot was unable to index your pages, another issue that is associated with the supplemantal index is that of dupe content. (A lot of scraper sites seem to appear in this index - whay-hey)
Whatever the case Google sees this as an auxilliary index and will only use it if there are not enough results in the main index to fulfil a specific -hard to find- query.
If your server was down temporarrily there seems a good chance of recovery, if not no-one seems to be giving examples of recovering from this index.
"cache date said Dec 31, 1969": I believe the day before the first unix system date and sometimes used by engineers to tag or id files. This may also be the case with Google engineers.
The problem has aslo been associated with "results crowding", the &filter=0 issue, and recent google screwedupnesses.
No-one seems to be able to put their finger on it exactly.
This is all just off the top of my head and I'm quite happy to be corrected on any of the above.
Another good way to get into the supp index is to have too many dashes in your domain name (3+) AND not add any content to the site for awhile or gain new IBL's, etc. Orphaned pages also go there frequently.
Not a fun place to go...