MW single hung windows-Installed in new construction in 2001-No other markings on hardware

Thank you for reaching out to us. It looks like you have an old rod balance that's no longer made. Is the diameter of the spring 1/2"? If so, I recommend the S795-28 with the 28C stamp option. A pair of balances with this stamp will support a sash weighing 18 to 23 pounds. Note that the Series 795 comes attached with the 1-1/4" wide 15-195 shoe, so you'll want to make sure the old one measures the same. I also recommend watching the video below. It will go over everything you'll need to know. 

Please let us know if you have any questions.