I believe that's the retaining wall and the hedgerow that runs in front of it.
I'm going out a limb here, and bet that you haven't mapped any of this out. The photographers location and line-of-sights onto a map.
The SS/FBI was at Dealey Plaza on at least 2 separate occasions/points in time doing their filming recreations Prior to the Warren Report being completed. And their separate Dealey Plaza appearances were Not a day apart, or anything short term. I believe your bottom image is from a far different point in time than the source I cited which clearly displays a TARP/TENT. I also believe I have discovered images which reveal the manner in which that TARP/TENT was delivered to Dealey Plaza. Getting a TARP/TENT of this size inside Dealey Plaza had to be done covertly and quickly. Your posting leads to the assumption that both images were captured at roughly the same point in time/SS visit to Dealey Plaza. I believe your bottom image is from a different SS visit to Dealey Plaza vs the Top Image. There is no mistaking that TARP/TENT for being the Retaining Wall. I believe the TARP/TENT stretches across the cement/concrete walkway which leads from The Shelter to The Steps. It also extends a little Beyond the "hedgerow".
