Malcolm's correct in his assessment if I am following you two (Richie and Malcolm) -
The main structures of the Electra were aluminum. Richie, you noted aluminum 'skins' - correct - but so were the frames in the interior picture (aluminum) including the main spar (a truss affair built up of formed aluminum plate).
The engine mounts - tubular structures attached to the nacelles out on the wings - WERE steel, as would be the major landing gear components. Not much else of 'steel' in there, however.
As Malcolm noted, you get issues when you mix the two - differential metals set up a difference in potenti which amounts to corrosion when exposed to the right conditions, especially contaminants and moisture (acts like a battery). Care has to be take to isolate the two when they do interface in the construction of an airplane, e.g. where a steel mount bolts to an aluminum structure.
Hope that helps clarify a bit. Interesting pix from those bulletins. Read those carefully - they may reveal more about the point I am trying to clarify here.