I saw the innards of a door like that once. I could see how corrugated cardboard of the right type would be good enough to stiffen up a light weight interior door.

I've encountered some door locks like that that should have been a lot easier to work than they were.

If I didn't want to sound like a dummy I'd guess that that's some standard poodle action.

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1986 300SDL, 362K
1984 300D, 138K