What is this Google Operator code?

Not sure I remember seeing this before

[Scoreboard]

Let's say you search for microsoft.com on Google. You'll then be shown the usual:

Show Google's cache of www.microsoft.com
Find web pages that are similar to www.microsoft.com
Find web pages that link to www.microsoft.com
Find web pages from the site www.microsoft.com
Find web pages that contain the term "www.microsoft.com"

Now, when you I click on the "link to", rather than seeing "link:" info, I see randomly assigned strings before eg. "Results 1 - 10 of about 129,000 linking to VRA9OSkpgNsJ:www.microsoft.com/"

What is this "VRA9OSkpgNsJ"?

[g1smd]

It is the Google DocumentID. Every page they store in their cache has a unique reference "number".

For any one page that ID will change from time to time whenever Google stores a newer version.

[g1smd]

... and by chance all the cache links for documents that I have viewed in recent days are now light blue again: that either means that they dropped off the end of my stored history (unlikely, the history is set for 10 days), or more I believe that the actual DocId has changed for those pages, even though many are showing a cache date dated from 5 to 15 days ago.