This is a very interesting thread. The mention of the classification of the keystone compared to the map and legend is off though. HOWEVER, it triggers a very interesting thought that I'm sure someone else had already.

Legend and map are unclassified. Keystone is (I think) unclassified. Put them together (legend and map with the keystone joining them like an actual keystone) and they become classified.

This is known as "Derivative Classification," where unclassified information, aggregated or assembled in a way that produced Classified information, even though classified information was not used to do it, make the assembled information, in fact, Classified.

It seems like an indication that the information and key to putting it together is all public information.