That looks like a temporary plug for that opening that was used only during production. If could be that a lot of manpower was used to move the fuselage along the assembly line. They may have preferred the door frame to be the hand-hold of choice and not the window frame which would be much more fragile. The plug would have been removed just before the window was installed. We have already seen evidence that electric drills were being used. It would have been tempting for the guys on the assembly line to pass the extension cord through the window opening instead of through the door. However, the window opening at that stage may not have been extension cord friendly. The solution would be to install a temporary plug. You see this kind of thing all the time today. Temporary caps, plates, plugs and covers can be seen everywhere on today’s assembly lines. It would not be a stretch of the imagination that they were in use when the Electra was being built.
Stringers adjacent to the windows …
Note that the window belt will have more longitudinal stringers, or intercostals, around the original window openings. If a fuselage was to be built without windows in a particular area the additional intercostals would be left out to simplify construction and save weight. If an opening was added at a later date this additional structure would have to be added.
I can't recall if Tighar has access to an SRM for the Model 10. If you do it might have a typical stringer diagram that might prove useful. It might also have a skin, or plating, diagram. The gold standard would be the actual production drawings, but, those may be hard to find.
Back to the patch …
What keeps bugging me is there is “no window”, then a “big window” which gets replaced by a “big patch”. In all probability the patch was a field repair that was performed by a local mechanic using the standard practices that were in use at the time. I’m a somewhat convinced that the window installation itself may not have been done by Lockheed. This window-become-patched-opening looks to be larger than a typical window. At least one more stringer would have to be cut and would require more structure to restore the load path around the opening. If we were doing that today you would probably see an external doubler applied. I don't see any evidence of an internal doubler or structure. This may be due to the low resolution of the photos we have available. An internal doubler could have been installed, but, this would have been a far more extensive installation since mold line of the skin would be stepped inboard by the thickness of the doubler. You would have to remove and replace at least the frames on either side of the window opening and a bunch of the fasteners in the stringers. I just cannot imagine them doing all of that. At the same time there is no evidence of an external doubler. A convincing argument could be made that the fuselage at the location of that window could have been weak enough so the flex experienced in a hard landing could have cracked the window. It might have happened more than once --- hence the patch. That piece of acrylic would have to be specially formed to match the fuselage contour at that location. A patch just attached with fasteners around the periphery would have to be stiffened to keep it from oil canning. The mechanic may have just looked at the rivet patterns around the other windows and used that as a rough guide. One stiffener would probably have been fine. He may have had to use three stiffeners, along with some “hand forming”, or bending, in order to get the “patch” to lay as flush as possible to the skin around the opening. The result would have been somewhat rough.
I can’t figure out why they needed such a large window at that location. The better question woud be: If they thought that he large window was necessary, and obviously they did at some point, why would they have attempted the round-the-world flight without it?