Now...I don't know much about etching, but are they not done in reverse, like woodcuts ?
This image seems to cover a lot of the points when flipped over. Of course a lot of bluestones have been re-erected, so could the two spectatoprs not have been sitting on the now standing stone by stone 53?

http://www.stonehengelaserscan.org/images/stonehenge/360_still_sm.jpg