That doesn't explain why the Washington Post hasn't released their copy of the slides, though...those two outlets are competing on this story (it's nice to have two independent established outlets compete on a story of such national and specific importance, fwiw) and the WaPo could have the scoop. Maybe they're waiting for the Sunday edition?

My guess was that the source feared that one or all of the slides have some kind of identifying tag, if not as a meta-watermark but as something tell-tale in the content...and so had requested the Guardian and the Post to release as little as possible.