This is the SWISCO Series 775 Channel Balance Sash Support System (also known as a block and tackle balance) for tilt-in windows.
These balances have a metal channel that measures 9/16" wide and houses a spring and pulley designed to raise and lower the sash. Comes fully assembled with a 19-036 top mounting screw support and a bottom attachment called a wedge, available in 1" and 1-1/4" widths.
