Question about past remodel.

The removal of wallpaper has lead to a closer examination of other areas of the back entrance foyer.  Above the door that leads to the basement stair access, is a window.  Strange place for a window, since the servant's staircase forms a ceiling that is just behind the window.  The window was not noticed at first, because the window frame and glass had been painted gray.  Why?  Perhaps painting the window gray hid the fact that old wallpaper on the ceiling behind the window was hidden.  Exploring further, I removed the window by pulling out the window casing.  Two things came to light.  First plaster covered part of the frame, which showed old plaster repair or the casing was installed very early in the life of the house.  Round nails indicate that the frame above the door as built later.  When the window was loose enough to remove, I found that it was the bottom sash from another window.   The angled top to the sash was designed to seal the joint with the upper sash in the original window.  Why had this window been placed above the basement door?  Perhaps to seal out "Vapors" from the dirt floored basement.  What was above the door before the window was installed?  For that matter, is the door below the window even original?  When and who are the next questions that come to mind.  Last of all, there is the question "what to do about the old salvage window above the door."