Weird rust-through on driver's door shell

I took the door panel off the driver's side of my '83 400i yesterday to fix the door handle linkage and found this very severe and very strange patch of rust on the shell, almost at the rear of the door. Check the photo. What makes it strange is that it so high up on the panel, not down in the bottom of the shell where water and debris might be trapped. The rusty area ends at the point where the shell panel goes from vertical to horizontal. Conversely, the bottom of the shell is rust free and clear of debris, and the drain holes are unobstructed. There is no water staining at all on the inside of the interior door panel in the location of this rust patch. Yet the rust obviously began on the outside of the shell, on the surface which faces the door panel. The patch is much larger on the outside than on the inside.

I applied a wire wheel and some P.O.R. 15 and I'm going to put the door panel back on and forget all about this rust patch. But I'm curious if anyone else has observed this strange condition. BTW, my car has some very minor rust bubbles at other locations, the kind that only the owner can see, and some extensive surface rust in the trunk under the backlight. I haven't looked into the trunk too closely, a mixture of cowardice and a long list of higher priorities.
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I like the thin primer theory. Bare steel would rot in 29 years even without any obvious exposure to water. Here's a pic from today, after two coats of P.O.R. 15 and with a sheet of white paper inserted behind, to show how extensive the rust-through is. Uuuugly!
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In my first photo we can see, on the backside of the outer door sheetmetal, a lumpy material which may or may not be properly described as "undercoating," and which is apparently coated with primer or some other coating which looks not to be suitable for a top coat. We also see, on the door assembly's inner sheet metal, what appears to be a coating of my car's top coat, Rosso Nearco, applied to sheet metal which does not have the lumpy material. However, because primer often comes in a red hue -- e.g., the color of the backside of the outer sheetmetal on this very door -- this paint may also be primer and not the undoubtedly more expensive Rosso Nearco top coat used on the exterior.

By the way, I hope I can be forgiven for not doing a repair of this area and/or undertaking other restorative and preventive measures at this time. Job One for this car is to get it past the California Smog Check II process and registered for the road. With limited weekend hours available, at this time I have to target my efforts at that goal. Cali cops -- and our motor vehicle regulations -- can be such a Gestapo operation that a single traffic stop could lead to an impound and other unpleasant bureaucratic processes: things I would like to avoid.