Although I can well believe that such things did occur, he's actually quite a long way from the Sphinx, which is a big object (66 feet high, according to wiki). That looks like a scratch or mark, probably on the original negative or occurring during processing. You're right about the pose suggesting a Box Brownie Scurv!
Speaking of bad acts and the Sphinx, my wife's grandfather allegedly liberated a chunk of the Sphinx's nose during WWII (he was with the 8th Army). If true then the piece of coarse sandstone that he brought back that is sitting in the drawer will remain firmly under lock and key..........