Looking at numerous pictures of the doors (versions /A to /C) did not solve all questions.
When you have a picture that shows detail it usually only shows one door and/or one side.
It therefore stays tricky to determine what is standard, and what is field modification.
Many reports mention problems keeping the doors shut.
On the manual pictures I see the biggest question. There is no latch on the "co-drivers door"!
Pictures showing a latch could show a field added feature.
From the 1942 manual a pre delivery 251 (no equipment and seats installed) shows the locking of the rear doors.
A part of a "family snapshot" shows the locking from the side. The construction is fairly simple. There are several reports of bolts that are used to keep the doors closed.
This simple but good drawing (not to scale) was drawn by Bruce Culver. It was intended to make new doors to the Tamiya 251/C. Don't use them just like that, for that body is too narrow and the angles of it are wrong.
I had to research that for my kit (conversion to make a welded /c) says Walt Stallings. On the drivers side door there is a large pull handle attached to a small triangle piece of steel. There are two rods connected the triangle piece of steel. The two rods are activated to push out to lock the door and pull in to open the door. On the outside of the same door is a latch like that of a gate. There is a cut out to go over a rod like the gate latch. It moves either up or down (straight across) when the door is open or closed. On the opposite door, there is a welded bolt or a stud. The stud lines up with the latch from the driver's door. I have pictures of the doors locked and the latch is down over the welded bolt. The same book shows the doors open the latch is up off the bolt. On the passenger side door there are no other parts except the hinge itself.
I think that the large door handle is connected to the triangle metal part. The two rods that lock the door are connected to the triangle. The pull handle is in the closed position pointed down if the pictures are correct. A crew member only need pull up the handle and the rods retract opening the drivers side door.
As for the other side door, I see a latch on the outside of the driver side door. The latch is always up when the door is open. If the doors are both closed, I see the latch is down over the welded bolt on the other door, I am guessing that the handle and outside latch are connected together. Could the handle rotate, say clockwise to open the latch and counter clockwise to shut the latch? The motion would be quick enough for the crew member to pull and turn the handle to open both of the doors.
My sources are Sturm and Drang No3 SD.KFZ. 250/ 251 and the Ryton Schützenpanzer book.
An interesting drawing and a good suggestion came from a re-enactor. The doors are that heavy that a angled edge could never hold them shut. We found an easy accessible solution by this simple latch on the outside. You simple bend over from the inside to open the latch.
Several solutions like this are actually reported on war-time vehicles. They often are simple, even crude solutions.
This handle on a 251/1 with a special flak gun of the Grossdeutschland division gives a good example.