28 inches long 1/2inch by 1/2 inch square channel

Thanks for reaching out! It looks like you have either the S351-28 with a 1" shoe or the S390-27 with a 1-1/8" shoe. To determine which series is needed for your window, look over the dimensions from the diagram below to confirm the depth of your bottom shoe. 

For your 27C stamp, you should be able to use the 2730 stamp option which can support a sash weighing 13 to 19lbs.

Comparison of the S351 and S390 balance shoes.