My guess: A photo of some object that the U.S. military doesn’t want anyone to see so they covered it up while it is being transported.
If it were me I would stick a few funky braces around here and there just to confuse the enemy and the 13 year old kids and Freepers who are trying to figure out what’s inside.
They trucked the A-12/SR-71 out from Palmdale to Area 51 and Edwards in comparable “boxes” after removing the outerwing tips.