Milgard windows . Has number 2640 on balancer. 27 " long.

Hello, thank you for posting!

It looks like you have either our S351-27, with a 15-002 bottom attachment, or S390-27, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. You will need a 27" balance with a 2640 stamp option. 

For the balance that is stamped with 2690, you will need to obtain the weight of the window sash and compare to the strength chart on the product page to choose the correct option. The 2690 is an older stamp and does not match up to our strength chart. Please let me know if you have any questions. 

Comparison of the S351 and S390 balance shoes.