Channel Window balance - please identify what I need to replace these!

Stamped on the side of my 2 channel window balances (see photo) is the number "2740", along with "BSI" and "BJ". The length of the long metal u-shaped channel is 28 inches. Also attached are photos of the 2 plastic end pieces. The grids on the underlying graph paper are 1/4 inch .

The pulley that attaches to the large spring has pulled apart so that the spring is no longer attached, on both of my balances. I would like to replace both of these channel window balances, with the plastic end pieces already installed. Can you give me a quote to include any shipping and tax?