Need a couple new window pieces with the strings.

Hello! This part is called a channel balance. Yours in particular appears to be a match for our S795-26.

Just note that our balance screws to the top of the window track, while yours installs with a hook. You can still use ours but you will also need to purchase our 26-200 separately. This pops into that square hole and remains in place, allowing you to screw our balance in.

Those represent the calibration of the spring. You can see a chart just under the product description that explains exactly how much weigh each option supports.

According to your initial post, you need the 27C stamp.