This looks like a 15-177. Measurements appear to be the same. The metal half moon part broke on both sides of the sash as you can see in the pics. Both springs appear in good condition. Can you confirm part #. No markings listed on it.
Thanks,
Doug
Paul from SWISCO responded:
June 24, 2016
Hello, Doug! I agree, the 15-177 appears to be our best match, especially if the dimensions are all the same.