It is to unlach the double hung window.

Thanks for posting. This is a surface lever for an internal tilt latch. The closest match we have is the 26-001. The size of the prongs and the distance between them needs to match for a replacement lever to work. See the dimensions of the 26-001 below to determine whether or not it matches what you have. 

Profile view of 26-001

Side view of 26-001

If this is not a match, you're probably not going to find it--internal tilt latches are hard to come by and have been phased out. Honestly you're better off seeing if you convert your window to surface mount tilt latches like the 26-228 and 26-229. The video below shows how to do this. Check it out.