Hey there!
It looks like you have either our S351-28, with a 15-002 bottom attachment, or S390-28, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You will also need a 2740 stamp option for this balance.
If you have any questions just let us know!
A stamp option refers to the stamp on the metal channel, which indicates the weight range your balance is calibrated to support. In this case, yours reads "2740", meaning that you'll need to select the 2740 option on the S390-28's store page. This option supports a sash weighing 19 to 26 pounds. Let me know if this makes sense!
