Channel balance 33 inches long 1/2 inch wide

Thanks for posting! Can you confirm the length of the metal channel? Based on your stamp of 3940, the length should be 40". 

If it is 40" long, I recommend our S395-40 with a 3940 option. But if it is 33", see our S395-33. It may have been cut down by the installers for whatever reason. You'd have to weigh your sash on a scale to confirm the best S395-33 stamp option.