Coil Balance Help to identify

You have an older style coil balance assembly that is discontinued and no longer made. The good news is that we do sell modern components that can replace it: see our 15-177 pivot shoe and Series 152 spring. 

It's odd for your spring to have a letter on it instead of a number; that's supposed to be the weight rating. "F" in this case might mean the same thing as a 6 pound rating, but I'm honestly just guessing. You may want to weigh your sash on a scale to verify. Assuming you have one spring per side, with 6 pounds of support each, that means your sash should weigh 12 pounds. It's worth checking.