Old style coil balance size "F"

Thanks for posting. This is an older style coil spring that isn't made anymore, but we should be able to substitute it with a more modern style. To know which coil spring you need, it would be best to see a picture of your pivot shoe out of the window. The style of the pivot shoe identifies the identify of the coil springs. I suspect you have something like our 15-177 shoe, which is compatible with our Series 152 Coil Springs. The stamp letter F may in fact translate to our S152-6, but it would be best to weigh your sash just in case. Each individual S152-6 can carry 6 lbs. At any rate, show us the shoe if you want to be certain. Many thanks.

Yes, this looks like our 15-177. So the Series 152 Coil Springs will work for you. Now, if you had springs with no stamps or wanted to replace all the springs, finding a way to obtain the exact sash weight would be best. Nonetheless, a coil stamped 'F' likely corresponds to our S152-6, for 6 lbs of weight support. 

Ah, let me clarify. Your coil springs are an older style that work with this shoe. The Series 152 Springs can screw to the sides of the 15-177 with a screw like our 33-072. Let me know if you have any other questions.