D. Scott Chatfield
The Oracle has come through again! Warren's close-up photo of 4260 shows some kind of fine screening on the side of the hatch. This wouldn't be necessary if the hatch was exhausting air. So it looks like they were drawing air into the carbody.
Keep in mind the main intake filters on a late F-unit are behind the side grille at the rear of the carbody. So this box was above and between them. And note the heavy lifting lugs. If this was just an empty plenum the original four lift rings topside would be more than sufficient.
The plenum theory also doesn't make sense to me because the exhaust stacks also draw hot air out of the top of the carbody. There is a 1/2" gap separating the manifold stack from the visible stack mounted on the carbody. The exhaust gas flow draws hot air inside the carbody out with it. You see this in all EMD non-turbo stacks.
So now the question is whether that NP B-unit has the same thing. If it is an aftermarket engine air intake filter it would seem the NP mounted it on the wrong end, since the dynamic brake is in the way. Or maybe B-units had the whole intake system reversed from an A-unit? Almost all B-units were gone by the time I went railroading. I'm not sure I've examined any that closely.
Also I don't think I see the heavy lifting lugs on the NP unit's box. Hmmmm....