From: https://www.facebook.com/photo.php?fbid=1600577373291101&set=gm.1183357248449966&type=3&theater
It definitely looks to be missing the rear quarter panel trim. I think it's a 9 passenger Custom series ; even has the poverty hub caps. Maybe a special or fleet order. No radio antenna either. Weird.
I don't think those are long distance transport trucks...more likely local/display/show/testing grounds stuff.??? It sure looks like alot of them don't have side glass.....can't see reflections anywhere. That middle carrier is all Mercs, right?...even the wagon which appears to have the same side trim as the Merc passenger cars.