WD-40 will clean the gunk out of it.
GM changed the direction of the inside handle link somewhere between 1948 and 1951. It was a safety thing so the door wouldn't open if on a sudden stop someone's knee pushed the handle, from what I understand. Why they didn't change the door panel recess at the same time is a puzzle, as the window operator changed in 1951 when the wing windows were introduced, requiring some changes to the interior door panel already.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.