39-091 roto window crank left or right?

The opposite side of the 39-091 is the 39-092. "Left" and "Right" handed operators are actually defined differently from manufacturer to manufacturer. Some companies define it by what side the operator sits on, others define it by what side the handle sits on, while others may define it by the direction in which the window opens. There really is no industry standard.

The best way for you to determine what operator you need is to compare your hardware to the pictures we have on our site. Does the arm face the same direction? What side are the gears on? What side has the bore?