Esteemed colleagues, 

My library has recently moved a number of maps from our off-site storage facility to our Central Library, and as a result we are interfiling our historical topographical maps.  While interfiling, we came up with a question we haven't been able to answer, and I'm hoping you can help.

We have come across several pairs of 7.5 minute USGS topographical maps that appear to be duplicates.  They have the same original date, the same revision/photorevision dates, and the same notations in the lower left corner about source data etc.  All the information around the edges of the maps looks identical, and the maps themselves also look identical, so far as we can see.

But there is one difference.  One of the pair of maps has some dots just below the map image, on the right side (near the statement "Interior - Geological Survey - Reston Virginia").  The second map in the pair has no dots -- or sometimes the second map has a different selection of dots.

The dots we've seen are black, green or purple.  Sometimes there is a black star next to the dots.  I've attached a detail photograph of our 1969 Boyds, WA quadrangle sheet, which has a black star, a green dot, and a black dot.

The purple dots seem to be present only on sheets that have been photorevised, with photorevisions noted in purple.  But we haven't been able to puzzle out what the other dots mean, and a careful perusal of our copy of Morris M. Thompson's Maps for America did not reveal anything.  

Can any of you help us out?  Or, can you tell us where we should look to find the answer to this question?

thank you!

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