To me (I'm certainly no expert) the absense of the stringers is not important. Why? ... because they're absent! No matter how it happened, it DID happen. So if this is off another plane or whatever other theory you can come up with, it obviously once was riveted to stringers, and now it's not, and that happened on a deserted island. How that happened has no bearing on whether or not it was the patch panel on the Lockheed. It also could have happened in 1990. I say that in jest but the point stands.
So with that out of the way, another major problem that comes up is why would they have installed a stringer at the bottom of the patch panel, when there's clearly a supporting stringer there already? My personal opinion (just speculation) imagining the situation if I was doing the repair would be this: The bottom stringer was double riveted, and when drilled out, may have been damaged or looked bad, or looked weak with the number of holes, so it was simply decided to add an extra stringer for 'fresh meat' so to speak to rivet that patch to at the bottom. If you look at the beautiful photo of Amelia standing next to the window, you'll notice that bottom support had already been 'swiss cheese' 'd as a previous poster eloquently described it. The top rivet line being so close to the top of the window isn't a problem for me; the bend was more severe up there and that could have been to help curve it in as suggested by the kind metalworker a few pages back.
My final problem I see with it is, how did it survive but nothing else on the plane did? I immediately would assume somebody literally kicked the piece off the side of the plane, as others have speculated. I'm just a good old boy and will tell you this, I feel I could give it a good go and might not have much of a problem at all kicking that off the plane. If I was stranded on an island I'd be trying to steal everything feasible off the plane, and a piece of sheetmetal would be attractive... especially one with a nice frame around it that I knew had been riveted on top. It would almost be like a bullseye for my foot.
It may have been easier to remove than even my John Wayne alter-ego can imagine... and there's something to be said for the fact that in general a full grown man 80 years ago was probably more fit than I am sitting behind this computer. I feel I could kick the patch out fairly easily, and I also feel it would attract that if you were looking around the plane to scavenge things, and saw a screened over window opening.
Once again you come back to, too, it ripped off of something. Whether or not it was this plane we can't prove right now, but it was removed, jaggedly, bluntly, from *something*, and then had the stringers removed. You can't really argue, honestly, that since it was ripped off a plane, it couldn't have been ripped off the Lockheed. Of course it could! All of those rivets ripped through the skin, on a deserted island, one way or the other. I think the navigator could have done it, or Amelia herself could have done it no problem. what else are you going to do? After you've screamed for help for a day and then tried to kill crabs for a day you'd start looking at what you can steal off the plane before it floats off.
With all that said, I have no evidence to back any of it up, just an active imagination! This is not the smoking gun, it's just another in a long, long, long line of credible evidence that can be 90% proven to be what we suspect it is. When you add all of that up, I think you'd have to be a really dedicated critic to not make the small leap of faith and admit that it appears she actually landed on that reef.
Great work as always Ric, I've watched you on television and in news reports for years. Best wishes on the next trip, i'll be donating to help out.