No additional details that I can find

Looks like you need a new coil balance assembly. It's difficult to determine what you have seeing only the back side of it, but I think that might be our 15-180 pivot shoe. If so, this is compatible with the Series 190 coil spring, as well as the 15-186 detent clip and 15-182 barrel. All components are sold separately.

If there is no marking on the old coil spring, you need to get the precise weight of the sash in order to determine the proper calibration. Sash weight is split up among the coil springs that support it.