I have not heard the dirt street theory before, most of the time the theory is that streets were so narrow, that you risked getting hit if you exited or entered the truck on the street side. So you were expected to slide across and exit on the side next to the sidewalk.
I am not sure if a lock could be added - does the left hand latch mechanism have the locking feature like the right side? You cannot use the right side mechanism on the left.
