Simply dab some white paint on the center post of the rear sight. Sure, it's a little cheezy, but it gets the job done without all the stress of switching out the rear sight, if one can even find one for this old model.

I also suggest you snap up the magazines offered by another poster in this thread. They do not have that "bent and twisted" look of many used model 39 magazines when people tried to take off the base plate. Just some spray cleaner and a light touch of lube from the finger will take care of magazine maintenance. The plastic followers in the photos are the best kind, as they are less likely to bind.