Replacement piece needed in window balance

Hi Rebekah. That missing piece is called the metal cam. The nylon block it fits into is called a pivot shoe--that in turn is connected to that housing block with a coil spring above it, which is called the coil balance. This is what controls the window's movement. I am afraid that the metal cam is not sold separately. Moreover, your exact shoe is no longer made either--the 15-215 is the closest match, but we've found it does not interact well with the older coil springs. Because of this, you will be better off replacing the entire coil balance system in this window with the Series 140, which comes with everything I've listed: the pivot shoe attached to the coil spring and the housing that it sits in.

The video below shows how to remove and replace a similar coil spring system. Note that we generally recommend replacing both assemblies in the window at the same time. While this seems like a hassle, new and old coil springs do not mix well and tend to cause everything to fail faster, so it is for the best. 

To know what spring weights to select, weigh the window sash. The video below explains. Let me know if you have further quetions.