1/2" Channel x 29" long marking 28 3 Caldwell JM

Thanks for the photos. It looks like you have either the S351-29 with a 1" shoe or the S390-29 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. You'll need to select the 2830 stamp.

Comparison of the S351 and S390 balance shoes.