Here is what I think my door looked like originally. The triangular plate points up and matches up with a threaded mounting hole in the door. I can make the aftermarket mechanism work, but I would have to drill a 3rd mounting hole or just mount it with two bolts. My truck is a 1948 which I know is an early version and a little different than some of the later models (cab mounts, gas tank location, etc).
On a funny side note. My name is also Donaldo so reading the posted replies the first time was a little confusing and entertaining. What are the chances that two Donaldos both have old Chevy trucks and would be on this forum. Not a common name and I have not run across another Donaldo other than my father