Old andersen window balance

I'm trying to repair an old Andersen window where the sash balancer has failed.

I've attached pictures.

It appears to me the part numbers are 11600-17.
The windows is double hung

I see a number stamped on both balancers:

1031 BS27 4-91

My right balancer is broken. The left seems to work fine.

Could you tell me the proper replacement part number.

Thanks,
Dan

Additionally, this is the stamping on my left balancer. It's different from the right one. I need to replace this one. I may replace both so please give me both part numbers if they are different.

Thank You