Alenco no tilt balance 15D and 21C

I'm not sure what you have, unfortunately. We have dozens of different balances with those stamps. They're not product codes or anything, they just indicates the strength of the spring.

In order to figure out what Series this is, I need you to remove an undamaged version of this balance from the window and show me clear pictures of the top and bottom nylon fittings. I will also need to know the dimensions of these fittings.

You could have either the S351, with a 15-002 bottom attachment, or S390, with a 15-001 bottom attachment. Take a look at the image below to determine which balance you'll need. 

You'll also need 1540 spring strength options for your 16" replacement balances. You'll also need 2130 options for your 22" balances. Thank you!

Comparison of the S351 and S390 balance shoes.