Looking closer at the broadside photo of the MP gon, the door latches are plainly visible. The MP diagram says these are Wine.
But I've also noticed that on the Intermountain model, there are no latches. Were there some drop door designs that didn't need these? I'm assuming not, but I'd like to throw that question out.
Back to the prototype, there are four guys shoveling the contents over the side. Their aim was pretty good, since there are four discrete piles on the embankment. The doors probably wouldn't have been a good idea here.
After these 3000 cars were built in 1919, MoPac ordered 750 clones in 1925, with 10 drop doors instead of 8. In typical MP practice, along with the AB brake upgrade the cars also got power brake gear Moving forward in time, by 1949 a number of the original cars were rebuilt with steel sides. The last of them lasted to the late fifties.
Another note on the diagram says 'a few of these cars are fitted up to haul containers', meaning the small Youngstown limestone containers.