This is a Series 161 replacement constant force balance system for tilt windows. It moderates the operation of your window sash using a specifically calibrated coil spring that allows the window to stay open when raised and remain shut when closed.
This balance comes complete with a metal pivot cam, coil spring, and coil spring cover.
The spring is stainless steel and coated for low friction and corrosion resistance.
A pair of 3 lb assemblies will carry 4 to 6.99 lbs, (sold individually). The weight you need should be indicated as a stamp or imprint on your old balance cartridge cover.