Plastic insert

Thanks for posting! The piece you are searching for is the coil balance support. There is a chance that we may be able to supply you with a suitable replacement for it but I'll need to see a photo of the other compatible hardware to confirm the right replacement. Especially the coil spring.

You should also consider that the coil assembly might need to be replaced entirely or, at the very least, the coil spring. It should be connected to the coil balance pivot shoe which is typically connected to the bottom left and right corners of the sash.

Please let me know if you have any questions in the meantime.