Roto Window Slider?
A home owner from wisconsin says:
September 14, 2016
This part came off the bottom portion of the window, it slides into the metal guide you see. In the pictures I'm showing you 2 pics of the broken piece, and one that is still intact in the window. I'm not sure if you can tell, but it is the pivot point of the window.
Tom from SWISCO responded:
September 19, 2016
I'm afraid that black slider cannot be replaced by itself. You will need a new hinge. I recommend looking at our 36-013-10 hinge set to start. This may work for you.