This is the SWISCO Series 390 Side-load Channel Balance Sash Support System (also known as a block and tackle balance) for non-tilt windows. The series is determined by the combination of the top and bottom fittings attached to the metal channel. When determining length, only measure the metal channel; do not include the fittings in this measurement.
The metal channel's width is 9/16", but is considered 1/2" in the trade. It houses a spring and pulley system to raise and lower the sash and comes fully assembled with a non-winged 19-002 top guide and a non-winged 15-001 (1-1/8" deep) bottom shoe.
Please note that the Series 390 closely resembles the Series 351 channel balance, but they are not the same. The difference lies in the depth of the shoe: unlike the Series 390, The Series 351 balance has a 15-002 (1" deep) bottom shoe.