Channel Stamp ID Help Needed

Thanks for posting! If your sash is only 20 pounds, you want to go with the 3130 option. 3140 would match your old spring but my fear is that it will be too strong. You never know what the original installers were thinking when they installed your balance. Maybe the 3140 was all they had on hand and they put it in anyway, or maybe they thought stronger meant better, or any number of other reasons. 

At the end of the day, numbers don't lie. If your sash weight is exactly 20 pounds then you should trust that and forget the rest.