Broken part at top corner of sash

Hello,
In the process of replacing a window balance, I found this part at the top corner of the sash, that is supposed to catch the balance is also broken. Can you tell me what part this is, and how to order the correct replacement. The three included photos are: 1 of the broken part and 2 of the other side, unbroken.
thank you

Additionally, this is for a 20 lbs. sash, with 27" Series 385 Channel Balance
Option: 2640. Thanks!