Window Ballast Needed

Hey there!

Thanks for reaching out to us. It appears your top and bottom fittings are wingless. With that in mind, I recommend you have a look at the S351-22 and S390-22. The difference between these two parts is the depth of the bottom shoe as shown below. 

Your 21B stamp may translate to the 2120 stamp. A pair of balances with this stamp will support a sash weighing 8 to 12 pounds. You can always weigh the sash on a scale to confirm.

Let me know if you have any questions!

Comparison of the S351 and S390 balance shoes.