Considering the nose art is of the two characters climbing over mountains carrying gas cans, I'd say it was after they were in the transport mission hauling avgas over the mountains. (wild guess)
Karnak: Exactly. But if you look up higher, esso express and hobo queen also lack guns and are OD.
I'd like a little more info on this baby:

What are we looking at here? Are those engines and cowlings BMF, but the wings OD? The fuselage is clearly OD but with perhaps different shades (or metal?) rings around the base of the turrets and around the gunner windows.
What are we looking at here? Final paint? In-progress paint job? Assembly line quirk where engines from BMF met one of the last OD airframes?
Or possibly some very strange lighting/shadows affecting a paint job not fully "cured" thus making the engines and fuselage look different when the final product ends up being the same? (possible but I would doubt it).